Every expression pedal shell that I have received has come with a pinion gear meant for a knurled shaft pot. Crybaby-compatible pots have flatted shafts, so I have a ton of these gears that aren't being used. I remembered the many requests I have had for a greater range of pots that are compatible with an expression pedal, and I came up with the idea in this article:
http://www.smallbearelec.com/HowTos/Swell/Swell.htm (http://www.smallbearelec.com/HowTos/Swell/Swell.htm)
The biggest drawback of the potentiometer shown is that it does not have the double-screened track that you would expect in a guitar pot or wah pot. Also, it is single gang. If anyone knows of another off-the-shelf pot that might work for this application, do let me know and I'll try it out.