The tray will rust even faster now that your trapping moisture under the mat. You would be better off coating it with some form of bed liner spray.
You’d be better off just cleaning it up and hitting it with a rattle can every year. That mat will just trap moisture.
This will be great when the tank starts to cut into the mat and gets loose under the clamp. I think your way over estimating the problem you’re trying to solve.
I would probably just hit that tray with cosmoline or other wax film forming product, and maybe add a ring made of a length of split rubber tubing on the bottom of the propane tank.
Source: Put a rubber outdoor mat on propane tray to slow the speed of rust on the propane tray and tank.
