...(runs back to the computer) That was fast! I actually finished a half an hour ago, so it takes about a half an hour to do this mod if you take 3 tries to get it right.
I found the thick piece of styrene (about 1/32") to be too thick, but I had a piece of thin styrene...maybe .010" or so that I spray painted a black stripe on. I also shaped the styrene so that it would gradually and increasingly block the light source as the pedal was pressed.
The results were unbelieveable! This was what I was expecting when I bought the SQ5000! A nice, gradual build up in speed from 1 to the programmed speed over the full range of the actuating lever.
In the end, the thickness or translucense of the diffuser I made probably didn't matter a whole lot--I found that the diffuser I made was mostly the same shade of gray and that the gentile speed control was due primarily to the way I cut the diffuser so that it would more gradually (than the original) block the light the further it was rotated into the light sensor.
I know that this was the principle of operation of the original factory diffuser, but they got the shape wrong...That arc cut into the diffuser is much too sharp--it needs to be a long, gradual arc. I think you could probably cut the original diffuser so that it blocked light transmission more gradually, but I didn't want to test this out by making any permanent changes to anything because my servo motor is still under warranty.
Thanks again to Larry for cluing us in on this!
Spray Painted Styrene:
New Diffueser Stuck To Old Diffuser Using Ultra-Sticky Seamstick Canvas Basting Tape: