before cutting the fiberglass from the mold, you can lay stripes of foam "backer rod" (etc) on top and then lay strips of fiberglass over these to create a network of stiffening ribs.
If you would have left a 1 to 1 1/2 inch fiberglass flange around the perimeter, instead of cutting it off flush with the start of the parabola, it would have made for a much stiffer mirror base and provided you with a ready made mounting ring. You would then have a place in which to drill mounting holes, without compromising the integrity of your parabolic surface
Probably you are right. I tried to build an edge on the margin of the parabolic mirror (see template), but I failed. So I used a metal ring instead. Thank you for your advice!
A much cheaper and much less toxic alternative to epoxy resin is Titebond III waterproof, wood glue. It has tensile strength of 4k psi, which for low structural use is more than enough in combo with fiberglass.
