I got a small bracket made which utilised the strap which goes around the middle of the tank (I've got a couple of pics, not great, but might help). That bracket was then rivetted to the footwell.
Rob Morris my AO was working for Jaguar at the time and he then made me a plate/block out of nylon, which was about an inch thick, and it had grooves cut into it for the chassis rails so it did not slip about.
Others just put the tank on a plate and fixed it some way to those same rails.