Let me guess, is this going to be your first jet and you're trying to do it on a budget? I'm not saying that to sound like an ass, I'm saying it because if so then you're probably going to be spending a lot of money on something that will be dangerous.

Just using a layer of fiberglass over foam isn't going to cut it, not unless you use the right weight of cloth etc. It's much easier to sheet it with balsa first, then glass it if you want. But most important is going to be adding the correct structure. The goal of all model airplanes is adequate strength without a huge amount of weight. Just using foam with some fiberglass around it isn't going to do what you need. "Fireproofing" the area's where the turbine will sit doesn't mean anything. So you might put something there that won't burn, big deal, what will the structure be where the engine will mount, what is the structure in the area that will get hot? If all you do is put a piece of stainless steel over the foam structure so it doesn't light on fire, it's going to melt all the foam out and you'll end up with nothing left in there anyway.

If you give us a better idea of what it is you are trying to accomplish, perhaps we can give you some better idea's on how to accomplish it safely.