Math makes sense but the shape wouldn't look that much like a norman sheild. It would look really fat. I guesse I wasn't thinking about the 36" longest dimension issue. Anyways thanks for the help.
You have about 17"sq to play with, but, yeah, the 36" max dimension is going to limit your ability to tweak, unless you want less than max size. You could finagle it to be proportionally correct, but then you'll get less coverage.
Keep in mind to cut whatever will be the "body" of the shield (plywood, pink insulation foam, etc) smaller then that, so when you add the pipe foam around the edges it won't be over size.