PLEASE USE THE "ROTATED" PARTS OR ROTATE THEM YOURSELF IF YOU RECOMPILE/EXPORT IF NECESSARY.
I wanted to make use of some small motors from old electronics.
It turns out you can find at least two ~5V high speed motors in old CD drives. They have slightly different mounting holes (an yours may be completely different) but I've made this to fit both.
The design consists of a push (mallet) fit small gear which turns a larger gear secured to an m4 bolt. A second small gear secured to that m4 bolt then turns a second large gear, resulting in a ~5x reduction. I included the gear script by GregFrost that I used to fit the gears as well as the custom scad file I used to tweak them.
You should be able to assemble this with 4 m4 (624) bearings, 2 ~40mm m4 bolts, some m4 nuts and some ~40mm m3 bolts and nuts.
It fits well with (and I made it specifically for) my peristalitic pump (thing:409612).
Let me know if you have any questions/issues. The peripump definitely works, but I'm assembling the gear reducer for the first time now.