Get some foam that your resin won't attack, shape, glass. I use dvinycell sheet foam... It's not cheap but I like how it shapes and it's pretty dense so it has some strength. You can also use a pourable expandable foam... Box in the area, pour foam and carve with a steak knife. For the glass I use 6oz cloth because it's easier to wrap contours and keep air bubbles out. I have 2 layers of 6oz cloth over divinycell foam and have hit bottom a few times and have'nt cracked it yet. I left mine thin because I might do some reshaping. Ultimately I'd use @ 4-6 layers of 6oz over dvinycell and more over a foam that's less dense.