We welded sheet metal under the floor rails before we added the lower compartments. Looks like you already have the compartments in place so that will not work for you. You could tack weld in sheet metal in each hole or at least tabs to hold the insulation in place. In our first small build, we used tabs and used foil backed foam. Placed the foam with the foil down to stop water from getting to the foam as some of it was not covered by compartments. Ideally, you glue it to the floor plywood but that can be hard to get to. If you are real good at planning/measuring, cut out foam squares that fit the floor rail holes, glue it to the plywood then flip it over and put it in place!
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