I did this once on a smaller scale, making a skeletal hand rise up through a seonce table. This time, I wanted to go full-body ghost. There were a few different ways I could have gone with this, but I think it is important to have the ghost interacting with physical objects. So, in addition to the box, I put a candlestick on the box. As the ghost walks through the box, he pushes the candlestick out of the way. The flickering light from the candlestick also plays off the ghost, though you cannot see that in the video.
The first thing you need for a Pepper's ghost is room. That is an unfortunate cost as space is a premium in a garage haunt. The ghost needs to be a certain distance away from the kids, but the further away it is, the more room you need. Here is my layout:
The hardest part of this setup was to get everything to fit in a reasonable area, but still give the ghost actor (me) enough room to walk around but not be visible to the kids. That is part of the genious of the rotating bench. By restricting the movements of the kids, I also restrict their viewing angle.That said, There was still very little room for me to move around without a kid leaning forward being able to see me in the "ghost control area (area in blue in the picture below). So, I also put an arch in between the kids and the shrim to both partially hide the frame supporting the shrink wrap and to let me walk about one foot closer to the kids (area shown in purple).
The "ghost control" area needs to be a mirror image of the "ghost effect" area (the place they actually see the ghost) but rotated 90 degrees. I mapped out these areas and measured and remeasured the placement of the walls and the box so that everything matched up. Use a lot of masking tape on the floor. Some things are not so easy to move once you start building them. Let me first describe the ghost effect area then I will describe what I needed in the ghost control area.
By the way, the shrink film I used was a Frost King patio Door Insulation Kit (84" x 110") I ordered on Amazon.com for about $5 (order a few in case you burn a hole in one). The plastic was much tougher than I thought it would be. The only disadvantage of using the shrink film over a large piece of plexiglass is that you have to make a complete frame, the bottom of which is visible to the kids. I will take that trade off considering a 84" x 110" piece of plexiglass runs several hundred dollars.