The floor is glass. I'll be replacing it in the next couple of years to fit buoyancy foam so will fit doublers and stainless backing plates with welded nuts beneath the new floor. Will use toggle bolts until then and hope the floor holds; no great issue if it doesn't.
Straps are yet to be fully worked out. My general thought is to have bungee straps of the same length permanently fixed to both hoops of each seat frame on the gunwale side, with the strap length being correct for the slid-out position each side and adjusted to the slid-in position by simply winding it once around the walkway side legs to shorten it up - see the dodgy paint pic below. Hooks on the free ends for easy securing most likely. But I really just need to experiment and see how it works; winding the bungee once around the legs won't take up the full amount of slid-in slack so it's all going to come down to how stretchy the bungees are and how much sliding resistance they offer when in the slid-in, not-fully-stretched position.
When I do fit the new floor, I think I'll have it sprayed with Rhino Liner or similar over the gelcoat which would offer a fair bit of grip anyway.
Straps.jpg