I bought a different knob at radio shack, and it fits, and looks great!
It slipped right over the shaft, and has a tiny screw to tighten it in place. I used on a synth Filter module.
The only catch: the two knobs are close enough together that the "pointers" of each knob can touch if you turn them to face each other. Not good for playing with filter knobs in a real-time performance.
But there was a simple fix. The synth bit's knob as roughly a 270-degree range. Normal instinct is to orient "zero" to the lower left corner. The max "10" (or is it"11"! ha) would be in the lower right corner. My fix was to first orient the left knob as described, and then orient the right knob 90-degrees clockwise. Now the zero for the second knob is in the upper left corner, and each knob can turn the full range without touching.
I wish they had more colors, so I could use different ones on different modules, but this seems to be it. So, I'm still looking around for other knobs that would fit.