Here is a poor pic of the fuel tank support deck I made a while back – the bottom is 1 layer of 1708(+/-45) and a layer of 1400 (0/90) with a 3” strip of 9 oz cloth in the edges. The top is a single 1708. All vac bagged together. The edges of the ¾” coosa core are beveled from the bottom at ~22 degrees so the top and bottom glass comes together for 1” all around the core. The glass on the sides is also sloped at ~22 degrees to generally match the deadrise where it meets the hull down the sides. I shaped it 2" narrower forward so that it just barely touches the keel but will be epoxied down on the sides. It is pretty light and very stiff.
The original was just a ¾” sheet of plywood with some 24oz cloth slopped on it. It was not fitted to the space very well and was completely waterlogged (>30+ lbs even after sitting on a trailer for 10+ years)