This is a case designed for housing 8 Raspberry Pis along with a 140mm PC Fan.

The body consists of 2 Parts, the Frame and the Fan Cover. A Caddy needs to be printed for every Raspberry Pi that will be housed in this case. Expect a total Print Time of around 3-4 days at 80mm/s. Despite being fairly large parts, they will still fit on smaller Printers such as an Ender 3.

This case is designed to have every Pi individually removable without having to disassemble the entire case. This is realized using caddys with a compliant mechanism to lock them in place. This allows for removing Pis without the use of additional tools. Additionally, there is enough space to outfit every Pi with an "Armor Case", which is essentially just a big aluminum Heatsink for both sides of the Pi. This will significantly increase the surface area for heat dissipation, making the cooling fan much more effective.

Required Hardware

1-8x Raspberry Pi 3/4
1-8x Angled USB-C/Micro-USB Power Cable
4-32x M2.5x14mm Screws
1x 140mm Fan (Arctic P14 Recommended)
4x M4 Threaded Inserts (6mm Diameter)
4x M4x45mm Screws

Recommended Hardware

1-8x Raspberry Pi Armor Case
4-32x M2.5 Nuts and Spacers (if not using an Armor Case)
4x Sticky Rubber Feet (any size)
1x USB to 12V Boost Converter (for powering fan)
Multi-Port USB Power Supply (ensure >2A per Port)

Dimensions

Main Frame: 180x180x117mm
Fan Cover: 180x180x40mm
Pi Caddy: 107x65x6mm

Printing

This case can be printed using PLA entirely, since the oversized fan should prevent any significant heat buildup that could damage PLA parts. However, you should probably go for something more temperature resistant if you intend to overclock or really heavily load the Raspberry Pis.

Frame

This part should be printed with the Fan side facing down. Supports aren't necessary, but they can help to prevent sagging on the Holes for the threaded inserts.

Fan Cover

This part should be printed with the front side facing down. No further support material is required.

Pi Caddy

This part has a fully flat side that should be facing down.

Assembly

Assembly is relatively straight forward. First off, you need to embed the Threaded Inserts into the Frame. This can be done using a soldering Iron. Make sure you get them as centered as possible. If they are crooked, you might not be able to screw down the fan.

Frame

Once everything has cooled down, you can proceed to install the fan. This might be a little tricky. I suggest putting the fan Into the Fan Cover first and pushing the M4x45 Screws through the holes to align the fan with them. Once the fan is aligned with the cover, feed the fan wire through the Frame. Now you can move on to screwing the fan cover down. Align the Fan Cover with the Frame and make sure the fan wire is not getting squished anywhere. There should be a small gap for the wire to fit through. Gently pulling on the wire from the back of the Frame can help get the wire into a better position.

Pis

Now that the body is complete, you can move on to mounting your Raspberry Pis to the Caddys. The holes on the Caddy should align with the holes on both the Raspberry Pi 3 and 4.

If you are using an armor case, the screws that come with it are too short to mount the armor case to the caddy. This is why I listed M2.5x14mm screws as required hardware. These are just about the right length to get the Armor Case mounted to the caddy. Simply follow the usual procedure for assembling the armor case up until you get to the point of inserting the screws. Before inserting them, line up the caddy with the bottom of the armor case, making sure the USB/Ethernet Ports of the Pi are pointing towards the arms of the caddy. Now you can insert the screws and secure them, which should hold the Pi including the Armor Case together tightly. Keep in mind that due to slight differences in the placement of the USB/Ethernet Ports on the Pi 3 & 4, you will need a different model of the armor case in order to fit the respective Pi model.

If you're not using an Armor case, you will need M2.5 Nuts and Spacers. Start by feeding the screws through the bottom of the caddy. Once all of them are in, put some spacers on top of the screws. This is to keep some distance between the caddy and the Pi itself. Try to keep the Pi at about a 3-5mm distance from the Caddy. Once the spacers are on, you can add the Pi on top and screw it down using the M2.5 Nuts. Ensure the USB/Ethernet Ports of the Pi are directed towards the arms of the caddy.

Once you've mounted your Raspberry Pis to the Caddys, you can insert the Caddys into the Frame. This is also possible to do during operation. The right side of the case has an extra set of rails at the top. You'll need to insert the Caddys with the Pi facing down on this side. That way you will have enough clearance for the USB Power Connector on that side. The Caddys on the other Side should be Mounted with the Pis facing up. You will need to plug in the USB Power Cable into the Raspberry Pi before inserting the caddy.

Powering the Fan

In order to power the Fan, I would suggest using a USB 12V Boost Converter. This is basically a little circuit board with a USB Plug attached to it. There are two soldering pads on the other side, which will output 12V DC when plugged into a USB Port. Simply solder a 3-pin Molex KK (typical PC Fan Connector) to the pads, ensuring correct polarity. I would also suggest to wrap the whole thing in heat shrink to prevent any shorts.

Plug the fan into the USB Adapter you just made and plug that in to your Multi-Port USB Power Supply to power up the fan. You can of course use any other method you like to power the fan as well, such as a fan controller or a separate 12V power supply.

Of course, directly connecting the Fan to 12V will run it all full speed the entire time. While I do suggest running the Fan at full speed to ensure maximum cooling, you might be able to get away with a slower speed depending on your fan. A lot of fans will run at a slower speed if they are simply connected to a lower voltage, although it might make sense to use an adjustable PWM Fan Controller to get the Cooling to Noise ratio just right. Really, most 140mm Fans aren't terribly loud even at full speed, so you don't need to worry about it unless you intend to sleep in the same room as your Pi Cluster.

Finishing Up

After inserting your Pis with the USB Power Cables already plugged in, you can proceed to plug in any USB/Ethernet Cables you need in your setup. Once you've got everything set up, ensure your fan is running and plug in all of your Raspberry Pis into the Multi-Port USB Power Supply.

If you ever need to replace a Pi, you can simply unplug the wires for that Pi and remove the caddy without having to unplug any of the other Pis. The fan combined with the armor cases should keep all of your Pis very cool, allowing for maximum performance at all times while keeping the Pis in a very compact space.