I'm using matting board, which is the stuff used to frame the borders of picturs before putting them in frames. I get 3x5 (foot) sheets at my local Micheals for $7.99 a sheet. Of course, here in PA, there's a 6% tax, but I still manage to get a few sheets every so often. Then I'll print out the Pepekura, and glue or tape it on the mat board and cut it out with my handy-dandy X-acto knife. Matting board is VERY strong and durable. I pelted it with my airsoft rifle and aside from a few dimples, it was fine. Just go to the framing department of any craftstore and ask for a 3x5 sheet of matting board. They'll ask you if you want noramal or acid-free. get the normal, it's cheaper. hope that helps.
BTW you'll need a lot of sheets for MJOLNIR. save up if you want to use it. It's expensive, but worth it.
I'm using 110 lbs card stock and if you use hot glue like I am, you can make it pretty strong. But I don't know if it will be strong enough to hold up to putting on fiberglass. So thats why I'm going fill inside with spray foam to make it stiff enough for glass. Then after dremel the excess out.
For mine I used super glue to hold the flaps in place, then strengthened it by putting in a lot of hot glue in the inside. Why use tape???????? super glue drys as quick as it takes to put on a piece on tape and its as strong or stronger than tape.
Thus, resulting in a very strong model that is strong enough to withstand fiberglass, but thats after you have filled the inside with foam.