Another approach is the transparent plastic that moves between the LED and the LDR. I've seen this on some of the all-in-one floorboards. The plastic has a gradient that starts out transparent and ends up black. If you can figure out the mechanics, the gradients can be created and tweaked in Photoshop, then printed on clear film.
Very cool idea, using photoshop to draw the perfect taper.
Heres a couple pics of my Mu-tron Vol Wah it has an interesting optical mechanism, its a lot like the simple kind that use the shark fin with a V cut out for the light to shine through, but adds some lenses to focus the LEDs light I guess. It has an two LDRs in one lense, and in the other lense it has an LED, and a third LDR, that is there for some kind of current regulation on the LED (it seems unnecessarily complex, but they must have been thinking something).
I'm sure if you have access to a shop, a crybaby shell could be modded to have a similar metal shade. Maybe an IR LED so you don't have to worry about sealing the light out. High brightness LEDs are much more directional than older LEDs, that could help with keeping light from leaking around your fin too.
I have a curious old Edwards "light beam" vol pedal too, that one is much simpler, it just has a light bulb hooked up to AC, and has a plastic thing with a V shape and some semi transparent film over the V, thats attached to the rocker and gets between the bulb and an LDR.
With an LDR, you can tweak the taper by putting fixed resistors (or trim pots) in series or parallel with it, to help get the response you want.
We'd love to see a picture of your handmade pedal if you got one, I'm always impressed by that kind project.