[erebus]

I don't really remember how I fastened it.
I think I just used cleats made from starboard attached to the inside of the box, with strips of neoprene underneath.
Looked through my pics but didn't take one of the tank mounted apparently.
Poly fuel tanks only expand by about 3% so not a huge amount.
A tank 20 inches wide will only expand approx 1/2 inch to 3/4ths of an inch across its width.
Split up over the two sides of the tank that's only 1/4 to 3/8 of an inch on each side.
Not that big a deal.