OE gas tank “spacer”? Yea...step on in!

I would not think you want anything compressible. Maybe some rectangular aluminum tubing, say, 1" X 3-4" wide or so, front to back, both sides right inline with the straps. In fact, "I guess" you might be able to cut them long, with a slot/ hole in the end to index the strap and keep them aligned before tightening. Us thin pad material above and below the tubing