I used the felt originally, but it trapped water and started rust on the new boot floor pan. The next time, I used smaller pieces of foam. Better, but still rust when I removed the tank for cleaning last year.
I realized that nothing can prevent the water from collecting around whatever I put there. The solution is just to make sure the boot floor can't rust. I cleaned off the paint and put POR-15 on the area under the fuel tank, then roughed it up, primed it and painted over it.
I've got rubber strips there now.