Give Jon Goodman (Interslice) a holler at email@example.com his light seal kits always fit perfectly.
But, I agree with dehk. It doesn't necessarily have to be millimeter perfect. Generally the foam Jon uses for the mirror bumper is a softer open cell foam than that used for the rear light seal to help absorb to the impact. Heavy craft foam tends to be much firmer. With felt, I might be slightly concerned about the fibers coming loose and mucking about in the inner workings.