That's COOL.. When I had a sack in my E22, I considered making a shelf that would ride up and down on some stainless rods or pipe, so that it would rise as the sack expands. I was then going to make a ballast fill gauge and a switch to turn off the pump that would be triggered by the shelf.
To attach something to the hull, you can use liquid nails and glue a piece of MDF on there. Or, you can do it like they did it on my SX, take two strips of fiberglass mat and stick an ice cube to the hull. When the fiberglass cures and the ice cube melts, there's a raised fiberglass boss that you can put a screw through.
