I made this after seeing the Numberphile video 3 Gears are Possible. I saw his helical gearbox and had to have one for myself. This one is optimized for FDM printers. Simply print the following:
1x Gearbox Bottom @ .30mm layer height
1x Gearbox Top @ .30mm layer height
3x Helical Gears @ .10mm layer height
6x Twist Pins @ .30mm layer height
Apply a small amount of superglue around the edge of one of the pins, and put it into one of the helical gears AFTER putting it into the box. For the last gear you can assemble it outside the box, then put it into the frame and put the top on. Apply a small amount of superglue at the 4 corners of the lid to seal the box shut.
You can see a video of this in action here, it may take a while to wear in the gears to the point they turn smoothly.