I edited CharlieTCU's simple but excellent gearbox so you can stack them for a greater gear multiplier (or reduction if you go the other way) the result being a nifty go-fasty spinny thing. With a prop on the output it makes a breezy fan. Print everything at 100% infill other than the gearbox bodies (those work fine at 10%) for maximum sturdiness.
You will need one of everything except the following: 2x bolts, 2x spacers, and 2x top plates.
Hopefully you can deduce the assembly from the pictures. It printed with spot on tolerances on my printer, your luck may vary.