: I'll try thinner them out and see what happens

Totally remove the inside tape so the wood can compress as it flexes. Use heat and a putty knife rather than sanding. Normally wood compresses a lot more than it can stretch, so if you remove the fibreglass which is preventing it from compressing it should take a smooth and even curve. After the panels are bent you shape you can slap on a patch of fiberglass and resin over the joint, or a slice of plywood, (internal butt block) which should hold the curvature in place.

Have you considered that the areas behind these joints would be great places to install bulkheads? If you don't want to make them into airtight, or watertight compartments, you could dril 1 inch holes in the centers of the bulkheads. with two bulkheads you'ld have three compartments. Should one area crush and flood, the small hole, and trapped air, would limit the amount of water which flow into the other compartments.

If you don;t want bulkheads, consider putting in building frames, or simple thwarts -- a few sticks, even -- just to push out the area that need more "convincing".

PGJ

Recent news report said there was a Brian Cunningham at the South Pole with a kite-propelled sled made from high-tech composites. Thought it might be you