Like most folks, I was pretty disappointed by the ducting in the K40 laser. For one, the steel duct inside the unit extends too far in the working area and has a much smaller cross sectional area than a 4" duct pipe would provide. Additionally, you can't remove it without taking apart the gantry.

My duct assembles from the outside of the unit. All of the parts take M4 fasteners: countersunk to mount the internal duct to the chassis of the laser, and button head for the rest. The bolt flange goes on the inside of the laser cabinet and holds the nuts for you since there is no room in there to get a wrench once the duct is in place. I made the 90 degree adapter, but never ended up needing it. It should work still. It has some internal structure to help with printing it easier with fewer overhangs and supports. The internal duct and the 4" duct adapter should be able to print without supports.

Post printing note: Some lasers may have a bolt that supports the laser tube mounting brackets inside where the duct goes. I wasn't sure if this was standard on all K40 machines, or if the placement was the same, so I didn't really account for it. It's pretty easy to grind away a little plastic from the internal duct for clearance of that bolt head.

I tried my best to make the inside as aerodynamic and easy to print as possible. I may go back and edit the duct to be more compact if that's what people want. Let me know if you think something can be changed.

Including the Fusion360 file as well. Don't laugh too much at my Fusion skills, I've only been using it for a couple weeks.