I work in a large warehouse where we get all kinds of corrugated cardboard in. From 1/4" up to 6" thick. Most of my head stones are made of some very strong 3/4" and some 3". I'll cut them out. Use a glue gun to fill in the sides. Paint the background any gray, black or speckled paint I find at yard sales. I especially like paint called "stone". Then my wife will paint on the Halloween epitaphs. Some I thought of, some my wife, some a fellow jokester at work and others on Halloween websites. We will paint it with "black light" paint. After it drys I'll put a few coats of clear paint over everything. Under each headstone I'll drill two 6" holes up into the gravestone and hot glue 1/4" thick by 12" bamboo sticks into them. So 6" is into the gravestone and to set up I'll drive with a 1/4" thick steel rod two holes in the ground to slip the gravestone in with the remaining 6".
Then I'll hide about 10 florescent (18" to 36" long) black lights behind the fence I make or hide them behind other gravestones behind trees. I try to make it so a person cannot see the blacklights, so the gravestones really pop. It make it look like they are back lite or just glowing. I'll even paint the straw and leaves I place in front of each grave.
My last effect is the best.....It's hard to explain....At a garage a few year back I picked up this thing called "Lighting FX" What ever you plug into it will correspond to the spooky CD they give you. So what I do is I put 3 lamps into my RV trailer windows on that side. I put the CD into a remote cd player, I set my 2 JVC speakers under my trailer and the FX thing in front of one of the speakers. So if it is playing a lighting and thunder track, the lights in my trailer windows will react to each sound. Another track is squicky doors and loud footsteps...There is a sensitivity dial on the FX to adjust the effect. You can stand on the campground road and clap your hands and the trailer lights I had put in the window will react to each clap. Here is a Youtube of someones display using the Lighting FX thing
. I think new it was only $25...I got it for $5 brand new....
Or this one Youtube