Even tho its covered, I'd still use the inserts made for outside seat cushions. They provide stronger support in them than most foam by itself and they take the weather fine. If you want to add foam to it for a thicker/softer seat, you can do that. WalMart used to sell the cushion inserts in the craft area but don't know if they still do or not. I'd stay away from batting since, even tho it's covered, the area can still get wet from blowing rain and wet batting takes forever to dry. Even the foam collects water, but it's better than batting at getting dry.