I read a lot about a more rigid setup of the PLab spectrometer in this forum. So I thought about how to avoid the velcro- and paper instabilities and came up with this idea:
The webcam and grating foil holder would slide into the notches and keep the angles stable. You could build this from would or - as Warren proposed - laser-cut it from acrylic, which would cost more but be more moisture resistant. As for the DVD grating, I would exchange this against a ready made 1000 lines/mm diffraction grating slide that you get for instance at amazon for just $ 2.32. You could also use a 500 lines/mm or even 13500 lines/mm slide if you want. Somebody should be able to calculate which is best.
You can glue the side walls to the base block and add another notch slide for the slit in front of the spectrometer.
Oh, and has anyone thought about what happens if you dim the incoming light with two rotatable polar filters? Might prevent the webcam from over-exposure but I'm not sure if it changes the curves somehow. Could be worth a try.
And I still think it would be great if we could switch off the gain control of the webcam, too.