There are 2 basic way's to make tombstones and grave markers that look good and you can make them in great numbers.
for this you can use free box's that you find anywhere, just go to a store or industrial building and ask. i work in a warehouse so i can get all i want.
1)cut the front shape of your tombstone any design works.
2)cut out small side pieces for your tombstone and attach them with clear packing tape.
3)cut out a larger side piece that you attach to the bottom with the clear packing tape.
By now if you flip the front of it on the ground it should look like a hollow box
4)start painting the base color,you can go classic with gray and paint the side black or you can use fluorescent paint to make it glow under a black light.
5)to add letters and crack's or design's use a black marker and go nut's.
6)after your done set it down how ever you want it and set a brick or rock inside on that larger bottom side for stability.
Bamm ! Done!
1)get some sheets of styrofoam and cut out what ever shape you want, you could also glue two pieces together if you want it thicker.
2)same as before,paint it to your preference.
3)same as before, use a black marker to put letters cracks etc.
4)hammer a either wooden or metal steak into the ground and use clear packing tape and run it around the bottom of the steak and the tombstone, don't worry the tape wont show at night.
Bamm ! Done !
Hope this helps.