Two years ago I made a bunch of headstones out of insulation foam. This year I decided to make a grave slab which turned out to tall and is now a coffin. Anyway I used pretty much the same technique with this as I did the headstones. Only difference is the headstones did not need a wood frame.
Here you can see the wood frame made out of 1x2's. Each side can be taken apart to store flat.
Then I covered the wood frame with the foam. Wood glue and a few pressure clamps is all that is needed. I could have used Liquid Nails but didn't have any. Then the fun part. I used a hot knife, looks like an exacto knife you plug in, and start to add the detailing. This knife can be found at any hardware store. I advise cutting this foam with this knife outdoors. The fumes are horrible so wear a mask. The top is not attached at all. If I want it can slide to the side to look partially opened.
I also used white caulk to fill in any and smooth out any gaps I had. this also helps hold it all together as well as the glue. Caulk also works well for adding texture.
Then I primed it with a grey primer.
Once the primer was dry I just painted it to look old. It doesn't need to look perfect. There's a lot of splattering and dripping of paint to get the look I wanted. I also ragged on some paint.
And I'm done. Any imperfections in it won't be noticeable in the dark so I'm not going to worry about them.