Well we build our tombstones from scratch, so it's a little different. We sandwich pieces of pvc electrical conduit (which I think is like half inch pipe) between two slices of foam, and then we drive a couple piece of rebar into the ground and the PVC just slides over it. Works great for us because the stuff won't move at all and it actually makes the tombstone a bit more durable to have the PVC inside. I've seen people who just attach the PVC to the back and that works to although not as pretty from the back if that matters. I'd think the plywood would work as well - but I've never tried it.