I've used Fullerplast for years and it seems pretty flexible. I think one of the biggest factors is not putting too many coats on the limbs. I might have to put 15 coats on certain types of riser wood, but only use 2 or 3 coats on the limb glass.
I was using a Dupont auto clearcoat for a few years. I know they added a flex additive to the finish to keep it from cracking.