UPDATE:
v1.01:

• Added vent slots
• Improved catches
• Improved clearance for NIC ethernet cable removal
  Please let me know your results if you print it!

I designed this case to be a mini ITX pfSense router containing a discrete Intel NIC, an SSD and a slim 12cm fan.

It's quite neat and very easy to put together - only 2 halves that slide together - the front and the back!

Small printer friendly - each half footprint is approx 20cm x 8.6cm and height 11cm for the taller half.

Great as a small factor desktop, home theatre PC or a router/firewall.

The motherboard and NIC together slide into grooves, and are quite sturdy.

The the locking tabs might need some tweaking.

In my original V1 design, the tabs snapped off but the case is still usable. Since then I improved the design but haven't printed the new version myself so let me know your results.

It also has holes in the back for 2x SMA headers if you need to run a wifi module (may need to enlarge these - I didn't end up using them so can't comment on fit).

It's designed to fit a standard mini ITX backplate, so in theory any motherboard will fit provided the heatsink isn't too tall and the Pico PSU has clearance.

I used a 16mm LED push button as a on/off switch (overkill but looks nice).

Parts I used:
Asus J1900I-C Quad core celeron
http://www.ebuyer.com/696995-asus-j1900i-c-intel-celeron-quad-core-j1900-vga-hdmi-8-channel-hd-audio-mini-j1900i-c

12V Silver Angel Eye LED Car illuminated Momentary Push Button Start Switch 5A
http://www.ebay.co.uk/itm/191867967957

Intel Gigabit PRO 1000CT PCIe
http://www.ebuyer.com/148377-intel-gigabit-pro-1000ct-pcie-desktop-adapter-expi9301ct

Gelid Slim 12 UV Silent 120mm (overkill but the Intel NIC gets hot. It comes with a speed resistor, and is very effective. Silent and cool.)
https://www.quietpc.com/gel-slim-12-uv-blue

Transcend 370S 32 GB 2.5-Inch SATA 3 Solid State Drive (any will work)
https://www.amazon.co.uk/gp/product/B00VX82PJC

DC 24pin 12V Pico ATX PSU Auto /Computer PC MINI ITX Power Supply 160W Module UK
http://www.ebay.co.uk/itm/111377108201

• Generic ram

If you make one, please take pics!