Since the painting effort requires the protection and stability of the material the 3/16 tempered hardboard (Masonite)would be the way to go.
The stuff at the depot is a durable product and you could get it cut while you are there. Cut to the widest and trim to fit when you get home. It's available in 2' x 4' and 4' x 8'.
That's my 2ยข.