Here is an idea, you might hot glue two 10 to 14 inch pieces of 1/2 inch pvc pipe to the back of the tombstone vertical on each side and then you will have to find the the little yard signs with the metal legs, cut the metal legs with bolt cutters but you will want to leave the angle part at the top so when you push them into the ground they won't injure you hands. The angle part will also keep the tombstone from lifting off the ground and flying down the street. The pvc pipe can be found at any home improvement center and can be cut with a hack saw to the proper lenght.
The angle on the metal legs can also be used to pull the tombstone from the ground at the end of the evening so your display doesn't become somebody elses display.
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