I'm not an expert or anything, but are you just planning to lay the plastic over a mold and heat-gun it? With no vacuum/suction from underneath?
I don't think that will work the way you're expecting. A heatgun has too narrow a nozzle to effectively distribute heat over a large area, i.e. an armor plate mold. It's good for small areas, but will just heat and curl the plastic into some pseudo-Dawn-of-the-Dead bounty hunter armor. Without suction (in which case you're better off building a vacuform system), the plastic will warp uncontrollably. Give it a shot, but I have my doubts...
As for molds, the best (and cheapest!) thing I've found is Fix-It-All, found at Home Depot. Pours like Plaster of Paris, and dries solid as a rock.
Welcome to the group, by the way! You'll find dozens of threads for scratch-armor-building, if you look in the right places.