I am pondless at the moment. When I bought the property a few years ago you could poke a stick in the ground and hit water. I thought cool! My dad lives next door and has always had several sump pumps running in the basement. Two years ago, the ground water seems to have dried up. I am resigned to having to use a liner. The extra cost has pushed a pond on the back burner. What I have planned is to eventually have 3 ponds that are 30ft x 80ft x 8ft deep. One at a time of course.
I would just fabricate structure. An easy and inexpensive material to use is corrugated plastic:
Coroplast, Home Depot This material is easy to work with. If you use a pizza cutter, you can score it without cutting thru to easily bend it. 10 sheets for 110 bucks, which is pretty cheap. Coroplast is made from Polypropylene, and being corrugated, it is pretty strong for what it is. You can get it cheaper elsewhere, but you have to look out for shipping costs.
It should also work real good for building an RBC similar to Cecil's. Just cut little spacers from the left over material to space the sheets apart. It certainly will be quite a bit lighter than fiberglass.
You can get FDA approved glue's that are waterproof, so it is a very reasonable material to work with.
