The ETO gearboxes seem nice but they seem way to expensive to me for what they are. $265 and you can buy a whole tank.
I think the frame of an ETO-style gearbox could probably be 3D printed.
Can you buy motors and planetary gear boxes and rack and pinion gears from someplace to try and build your own?
Are the silver things at the front of the ETO motors planetary gearboxes?
Steve