That looks like the insulation that goes between the hold down strap and the tank.
My fuel tanks are held down by standard type ratchet straps, like the kind you get holding loads down on a truck trailer. They are widely available in lots of sizes and are low cost. I used a really cheap one about an inch and a half wide to hold my second calorifier down, and use 2 more for holding my tender in place against the vertical davits.
Cheap as chips, strong, unfastenable, what's not to like?