Unless you get really exotic, fiber glass wouldn't be my first choice for something structural. If I wanted light weight and strength, I'd be thinking a sandwhich or ribs.
Just as an example, a flat piece of sheet steel 2 feet square by 3/16" thick. With ribs of 1/2 angle iron spaced in rows. Three rows of angle iron will increase the strength drastically. It really doesn't have to be angle iron, ribs of 3/16" X 1/2" flat stock welded perpendicularly to the sheet steel will do pretty much the same thing. If you use two layers of sheet metal and ribs between the layers, you have really increased the strength, with much less weight.
I listen to a lot guys and they like to produce bullet proof stuff. My reasoning has always been, something is gonna bend or break and I'd rather it be my skid pan, than my Jeep. I usually try to build in some kind of crush or flex. One reason I prefer nuts and bolts to welding or both when practical. I also avoid solid beads when welding, I usually weld to 1 and 1/2 times the strength, needed to do the same job with a bolt or a series of one inch beads (spaced).