Tom,
I cut a floor (not part of the plans) out of some scrap 9mm to fit on top of the stringers in between frames A and B. I had the tank built before I glued the floor in place. I marked where I wanted my tank mounted, drilled the four holes for the 1/4" SS Hex Head bolts. I flipped the floor over, inserted the bolts through the holes, and cut some small pieces of plywood to make a 4 sided box around the Hex Head. Once the "box" was glued in place and the bolts were installed, I mixed up some epoxy with Silica and Milled Fiber and filled the box with and the small void spaces around the Hex Heads. Basically the box was glued in such as way to prevent the Hex Head bolts from spinning and the epoxy mixture was to hold the bolts from falling out.
-Reid
Here are a few previous pics to demonstrate: