Just figured id share an idea.
This was a few years ago so i dont have any pictures
I took foam and cut it into a tombstone shape and covered it with a thick garbage bag so no water got in. I tried the bag really tight so there werent many wrinkles and then sprayed it with the stone texture spray paint. It looks swesome and lasts the whole season.
If you're in a hurry like I was one year, you can make the stone out of cardboard & hit it with that same stone spray paint. It actually held up for years, masking and tape and all. Minimize rain exposure, but a little rain didn't kill it right away.