A couple days ago I saw an "ible" about building a cover for a fog machine that looked like a casket and of course now I can't find it ! If it's your "ible" let me know so I can link you. here it is ! http://www.instructables.com/id/Easy-Halloween-Casket-Fog-Machine-Cover/
That was the inspiration for the 1st open casket I built using foam board which I had recently rescued a large pile of from the trash. When my son in law saw it he suggested I make a couple for my daughters scare in this years haunt. After a little discussion we hit on a really gruesomely cool prop.
I'm not going to tell or show you here the final piece, you'll have to check out my Haunt "ible" after Halloween....
Step 1: Getting started
I started out with some large foam board sheets approximately 4'6x2'6 and cut out a couple of front and backs for a toe pincher coffin. I then cut a bunch of 4" wide strips. By the time I was done it was getting late so I went inside, when I came out the next day every thing had WARPED ! (I actually took ALL these pics after everything warped) I found a couple of heavy steel plates as weights and built around them