I'm going to have to build up the low side quite a bit (maybe 2+") and fit a backing block to an irregular shaped surface. Wood is probably easiest to work, but even if I used (rot-resistant) teak I would prefer not to have something porous between the thru-hull and sea-cock. So I'm thinking fiberglass, or some other material to mold the backing plates then epoxy them in-place (there are 3). If I did fiberglass I'd first take a mold using clay or plaster then use it form the fiberglass (or other backing plate material) off-site for subsequent final fitting/installation.

Any other creative ideas?