Closed cell foam filler pieces can be found at a local siding/roof panel manufacturer or even online. Just scribe the panel profile and search online.
at Lowe's for instance
I'm curious about adding a weatherstripping to the drawers of my RIGID toolbox as well. I will have mine outside from time to time. I like the simple solution of covering the box with a tarp or blanket. However I am looking into adding a bristle similar to what you find on aluminum storefront...