I don't think that clear plastic knobs will wind up looking too good - you'll see the bushing and shaft of the pot, plus the holes for the two set screws and the set screws themselves. These holes will be milky unless you have them buffed, which would be rather costly. Unless you can find a 1/4 shaft knob for the pots and a 1/8 shaft knob for the selector switch that is already made, having these parts custom made will probably be shockingly expensive.
Also note that knobs can be changed at any time! So if you start with a standard knob, and then later find the knob you love, you can always switch them out.
I know you're not going for a clear plastic backplate, but on that note, in case anyone else gets any ideas, that won't quite work because we need to complete the shield with the interior of the backplate, so if it were clear plastic, you'd see the silver paint through it.