I can get the two inch foam at HD. Depending on how you mount it on the ground 3/4 may work, but any amount of wind may cause it to break from the mounting steak. Thicker is better. For mounting I start with a 2x2x4-6 inch piece of wood. I drill a 1/4 in. hole in the middle of it. This is then glued to the foam board with liquid nail (special for foam). I then put the mounting steak in the ground and useing a nylon tie through the hole, I secure the 2x2 to the steak. This method worked well even in the wind and when it came time to take down the cemetry all that was needed was to cut the nylon tie and pull the steak from the ground. For tall tombstones the 2x2 was longer and i used two ties to mount it to the steak. I found this metod on somebodies page and I wish i could give them the credit due them. It worked very well for me.