Take a soft wood 2 x 4 and cut it to length for a sanding block, hand plane one flat side to approximately match the inside of the hull with the long direction in the direction of sanding. Place a full piece of coarse sandpaper in your hull grit up. Sand your planed sanding block on top of the sheet of sand paper until the block matches the curve of the hull. Glue the sandpaper of your choice (start with 60 grit) to the curved surface of your block with 3M Super 77 Spray Adhesive. Sand your hull. Use your heat gun to remove the used sandpaper from the block for replacement. Make several for different radius curves.

Or
Cut a piece of Styrofoam to the size of a 1/4 sheet (random) orbital sander about 1-1/2 inch thick. Shape the bottom of the Styrofoam with the face-up sand paper trick above to match the inside of the boat hull. Cut your 1/4 sheet of sandpaper extra long in the clamping direction of the sander’s base. Place the Styrofoam on the sander’s pad and clamp it tight to the sander with the sandpaper. Sand away. The foam will sand down with time from grit that gets between it and the sand paper, which is a good thing because it will adjust to match the changing inside curve of the hull. Clamp the sandpaper tighter if the foam starts to slide out.