Hey Blake,
I'm using foam core board from my local grocery store (~$4 per sheet), painted with a mix of spray enamel and stone effect paints from my local hardware store.
Ideally I'd like to do this out of plywood, but I don't have space for a woodwork shop
The foam core board warps when sprayed with the stone effect paint, as it is rather 'wet' and causes the top layer of card to swell and stretch the board. I've tried to coat the board with the enamel first to help seal it - it helps some but you still have to take care and build up the stone paint slowly.