If we had known we would be Halloween blogging, we would have taken better pictures and more notes while we were building our props. So, if you have questions, please let us know! :)
Depending on the desired dimensions of your pit, you'll need up to four sheets of plywood and at least two 8' runs of 2x4's. We cut the plywood to the desired size, making sure that the bottom length was the same as the inset length of the notch in our 2x4's. The idea is to create a friction fit so that the plywood slides into the notched rail in the 2x4 for easy disassembly and storage during the off season (if you have one at your house).
The "stone" is created using sheets of foam. We created the grout lines using a soldering iron - just heat it up and melt away. Again, do this OUTSIDE because even Haunters shouldn't breathe too much melting plastic. Glue your stone walls to the plywood.
Don't forget to cut a hole for your lightbulb and its housing in the plywood and foam. This will be attached at the end.
As for your top and bottom pieces - you want to create a notch that runs the length of the plywood sheets that is wide enough to shove (and I mean SHOVE) the plywood into. The inside of the 2x4 is also routered out to create a place for the mirror or Plexiglas to sit.
We like to use Plexiglas rather than a mirror so there is no danger of it breaking if some errant Halloweener decides to get a really close look at the bottom of your bottomless pit. For the top piece, you do have to epoxy the mirror to the 2x4 frame which has the same notches as the bottom piece. We tried to have it sit in a frame somehow, but the lip always showed when the Pit was lit up and ruined the whole effect.
Plexiglas is readily available and to make it "one-way" glass you'll need to go down to Osh or some other home retailer store. We found our one-way material in the window coverings section - it is really considered privacy material used on storefront windows. (Took us a while to figure that out in the store!) We didn't bother purchasing the specialty fluid to make application easier - we just used windex and it worked just fine. Application of the privacy screen to the Plexiglas is a two man job, so have help on hand.
We hope you like our project! The kids get a big kick out of it at our house.