Dave,
Sounds like a great way to make a block for a very specific area. How many would you practically need to do an entire kayak? The stem and stern change so rapidly that it seems like you would need a new one every 10". Practically that may be extreme.

: 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.