OK, couple of things...are you talking about making a mold out of hot glue? That's not a great idea if so. If you must have the least expensive material available, the caulking silicone you can buy from home depot or lowes is a better option. I know there's a huntorial around here on it somewhere, you should search it out. if you mean casting the bio in hot glue...why? It'll flop around like a boned fish.
Second, while it is possible to strengthen plaster of paris, I don't understand why you need to use it. Buy yourself a tub of regular bondo, the standard kind is fine. Then buy yourself some of the bondo fiberglass resin as well. Mix them together in a 50/50 ratio or something similar (you don't need to be exact, it just needs to be a thick but still runny consistency, and don't forget the hardener!) and then pour it into your mold. Slush it round in there until it sets, try and get an even coating everywhere. It will be heavier and somewhat more brittle than using fiberglass or regular casting resin, but it'll work, and all for an outlay of about $24. This is how most of the Halo costumers finish up their pepakura armor. Of course, the brittleness can be solved by using a lot more material, but then it will weigh a ton more...
If you're set on using plaster of paris, you need to buy some cheesecloth. Your local hardware store should have it. Cut it into strips, and mix up some plaster. It should have a nice thick consistency, still fairly runny though and definitely not lumpy. Dip your cheesecloth strips into the plaster mix so they're nice and soaked, and then lay them in the mold. Total cost on that method should run you about $8. If you use this method, congratulations, you will be the owner of the heaviest bio in the world.
Good luck with whatever you choose to do.