Slightly OT Fiberglass over foam problem

Hey guys, I've been lurking around here for quite a while to get an
education. I'm building a project that could use a little expert guidance.

I am making a paintball tank. Not the type that you put air in, but the type
that you put people in. This is my 2nd tank. I used plywood for the skin
of my first one. It was very heavy, and it didn't look very good. So, I've
decided to do a fiberglass over foam body on this one.

I'm not worried about weight as much as I am strength and cost. Hopefully
this thing doesn't fly when I'm done. I went to the lumber yard and
bought some 1/2" polyisocyanurate foam insulation. I removed the paper from
both sides, and put 1 layer of 10 ounce woven glass on each side using
polyester resin that I got from Wal-Mart. I just made a small piece (about 1
foot square) for testing.

I was very pleased with the weight of the product, and the look that it
would give my tank, so I took it out back, and shot it with my paintball
gun. The first shot went great. The stuff is much stronger than it looks.
But, if you pelt it a number of times in the same spot, the foam crushed and
caused delaminating.

Now, on to the questions. If I ad more layers, will it be able to withstand
the beating I want to give it? How many layers do you think I'll need? I
ordered some Vinyl Ester resin from aircraft spruce. Will this hold up
better? I've got somebody to donate some fiberglass mat. Would it be OK to
use this under the cloth for more strength? Obviously I'm trying to keep the
cost down. Any recommendations would be greatly appreciated.

Dave