we did some work at a gas plant and they wanted everything mounted on an aluminum backboard - got some 3/16 (I forget the mil gauge size) sheet from a local steel/alum furniture shop
Well thats kinda high for a 60 "x 60" sheet if it was 1/4 thick ?
Do you have a metal fab shop in your town we get our boxes & gutters made at a shop here local and a 48 "x 48'X 24" ' deep box only cost 800 bucks and thats with welded seams and cover with lock and a UL sticker .
Hoffman, Austin, and probably all of the enclosure manufacturers should have aluminum backboards that you can buy through your supply house. They are kinda pricey, but should be less than that 60" x 60" you were quoted.