I recently created some gears for my Ultimaker. These spur gears are created especially for laser cutting. Prefered material is POM (delrin). They work perfectly as seen in the video: https://www.youtube.com/watch?v=AAAizfJgaWM there's nearly no backlash at all. I was actually really surprised how well they worked. The hex nut can be pushed in with some force. This little gear is made for a non-D-shaped shaft motor and should also be placed by force. I cutted them in 3mm thick Delrin but 5mm should probably better.