Good question; I've seen a few with after market covers. Never ask where they bought it or if modification where needed.
I plan on covering my 64 swb with marine grade vinyl. My bed floor is shot, I've done several using this type cover, I use 1" L shape angle iron on the inner side rail, then use pressure treated plywood as base with 1/2" foam then cover with vinyl. The tailgate sort-of-locks the cover into the bed rails. Cheap and easy to do.
Just to give you some though....

Mitch