I am looking at creating 12" wide "planking" from 8' sheets with about 1/4" spacing in between. A 4 x 8 sheet of 3/16" (0.188") aluminum weighs 96 pounds. An 8 x 8 rack would weigh out a little over 200 pounds total.
The other option I am considering is composite decking. Some is actually a bit heavier than wood but there are a couple that are the same, plus no need to finish/re-finish, UV resistant, waterproof and one (ChoiceDek)that comes with a lifetime guarantee. Combined with aluminum supports it should save a few pounds and a lot of maintenance.
A couple of hundred pounds won't be that big an issue. It's the massive mongo timber, 1,000+ pound constructions I so often see on Skoolies that concern me. And unless the wood is properly treated & weatherproofed...a rain shower could possibly add another half-ton to the total.