I would get it slightly larger than needed, bend it right down the center into a "roof" shape (barely clearing the SS braid), and trim it to fit in the tun perfectly. This would provide less dead space, and keep more wort in contact with the grains than elevating the entire FB.
Keg #1: RHESB 6.2
Keg #2: Cascadian Stout
Keg #3: BMC West
Fermenter #1 Bourbon/Oak Vanilla Porter
Bottled: Mjolnir Mead, Lazy Daze Hefe, The Bollocks ESB, Dark Matter stout.