Working with fiberglass is exactly like paper mache. Just toxic.
Buy: Fiberglass resin, fiberglass mat or cloth (I would prefer cloth for this application), paint brushes, disposable cups, latex gloves.
Apply fiberglass resin (follow the directions on the can for catalyst to resin ratio) to the piece. Get it wet. Apply pieces of fiberglass cloth to the repair area. Paint resin over that. Done.
I suggest only putting it on the back since this sort of repair will most likely not look particularly pretty.
I suppose the easiest way would be to get some apoxie sculpt (http://www.avesstudio.com/index.php?...chk=1&Itemid=1
) then just apply it liberally to the back over the crack, and then very smoothly on the front over the crack to close the seam. In all honesty since the crack is on such a small piece I'd do it there. And since I assume that this is an Asuka chest piece it needs to be very flush to your chest for the plugsuit.
TLDR. Use apoxie sculpt. It's awesome.
The blade in this picture was sat on and broken, but fixed with apoxie sculpt and sanding.