I have one from gaugepods.com for my z31. Its a very pliable ABS type
plastic that would lend itself very well for this kind of fiberglass solution.
It's meant as a replacement for the pillar trim piece, but i found it also fits
over it. Molding with a heatgun made it fit really nice. Unfortunately a few
days in the summer heat un-molded what i had done and it resumed its
previously warped sides.
Here are some pics with my old autometer gauges installed:
Thanks for the post. I'd been considering doing something with fiberglass
before and its nice to see your solution.