Mounting the tank
My solution was to hem the tank in with braces--fore and aft, and to put a clamp over the top. I mocked up these pieces out of foam board and trimmed them to fit. The sides of the tank are not straight, so the cuts have to be curved to fit.
I then duplicated all of the pieces in pine two-by. I didn't even worry about using pressure-treated. If this area of the boat stays wet enough to rot the wood, I've got bigger problems than that.
I cut out a slot in the cleat that supports the V-berth plywood so that the top piece can slide down into place. The fore and aft boards are just screwed down to the fiberglass with self-tapping screws, but the top board is designed to be removeable.
At each end of the top board, I drilled a one-inch hole with a hole saw. I also took one of the plugs from drilling these holes, and cut it in half. The board is held in place by a #10 machine screw inserted from each end, through the half-round plug and held with a nut in the one-inch cutout.
The half-round cutout on top is because the board happened to land right below the finger hole of the V-berth plywood.
Special thanks to Mike Lehman of Gilleleje, #505, and Jim Mennucci of Quest, #433, who blazed the trail with the Kracor tank.