Why not a screw with a large fender washer?
I just buy rubber bungee straps in whatever size I need and do exactly that, bolt one end down with a flat washer and put a stainless S hook in the other end. For fuel tank or easy to access stuff I use S hooks on both ends.
And for what it's worth, I never used a strap around my water box in my Chan hull either. Once everything was installed there was no room for it to move, same deal with my Rok.