I'd suggest fiberglassing the corners and edges, as they tend to be the thinnest parts of the Styrene. Unfortunately, since they are the thinnest parts, they are also the most prone to heat damage caused by the fiberglass resin and hardener when they are mixed together.
Actually, the edges and corners are pretty tight on this mold. I was surprised myself. The bottom started to buckle and that got me nervous. I was just thinking of layering over what I have already, but I might as well mix the whole batch and do it all in one shot. Man this stuff reaks!!!
If you plan on using the OB1 tutorial,and making the PVC frame,and using the "Great Stuff" expanding foam it will be more than enough support for the jet pack.I suppose fiberglass will work,but you don't want to do any damage as mentioned with the heat from the fiberglass curing.Just looking out for you
Here's the way I fiberglass supported my jet pack. I made sure the areas between the seams where halves were glued together were layed down with fiberglass. I also reinforced areas where any contact from the front surface of the jet pack could pose a problem. For any of the areas in the jet pack that wouldn't be seen from the outside, I just left them alone.
I put two layers of fiberglass resin. I was planning on putting a total of three or maybe four layers. However, I'm thinking two or maybe just three would be fine enough.