I've wrestled with this idea for some time, what the factory used worked pretty well as there was little rust, none of which was serious, above the tank. I purchased closed cell foam in bulk for other applications & plan to use for a tank pad, it is water proof & has excellent sound proofing properties. I also observed that anywhere there was undercoat that there was no rust so my plan is to apply rattle-can undercoat to the bottom of the trunk pan where the foam will contact. Time will tell but I don't anticipate any long term issues with this two prong apprach.
Doubt if you'll get away with spending less than $20 in the end.
Check with your local upholstery shops especially those that cater to the marine customer.