I've got an older gear bag that does not have the plastic 1 or 2" bottoms that the newer bags offer for placing in the bottoms of boats to eliminate water issues. Does anyone have a suggestion for a product I can buy and then shape to fit the bottom of my older bag to turn it into a "boat bag"?

I can put grommets the bottom of my existing bag and run a flat head bolt into or through the hard bottom to hold it on, or that's what I'm thinking at this point. Any suggestions would be much appreciated as most of the new bags are quite expensive.

Thanks for your help,