I used some 3/4″ ply that had a nice veneer on it, because I had extra. I presonally prefer plywood to MDF. It’s a big easier to mill and doesn’t have the see super tiny dust (which I’m sure causes polio).
If I didn’t have anything, I’d find some pine plywood. You want the spoil board to be softer than your material and you want it as flat as possible and reasonable (so no OSB).