i came up with this idea while driving home this afternoon, it may be kinda stupid but for something simple i can't see why it wouldn't be any good except for being a little bulky.
i have drawn up a rough image in ms paint but basically on sound boards and other similar devices they have variable resistors that are sliders, my plan uses 2 of these for each finger but basically when you clench a fist your fingers pull the mounts with them which pulls the pull/push rod which pulls the VR hence changing the output value.
the push/pull rods should be aloud to pivot also
I'd mount this onto a glove and for simplicity you'll probably only have 2 fingers on your arm so I'd stitch/glue all the fingers together and just use one on the 4 fingers and one on the thumb.
anyway, just a sugestion