Making an electric chair was cheap and easy for me. I had all of the stuff I used laying around the house.
First you will need an old chair. Take and attach a variable speed ( has a lil' nob to increase/decrease the speed) drill to the bottom of the chair, with the chuck (thing the bit goes in) facing out towards the back. This is done by attaching a piece of wood to the bottom of the chair, and then hooking the drill to it with a large hose clamp, or with several large wire ties. Set that aside for now. Now cut a circle out of ply-wood, this will be used to move the dummy up and down in the chair. The larger the circle the higher the dummy will rise. I used a 2" circle. Next, take and drill a hole in the center of the circle, to put a bolt in, that will be attached to the drill and another one somewhere on the outer edge to attach to the arm that will move the dummy (refer to figure #1). Now take and attach the 2 bolts to the circle and tighten them down with the lock nuts. Make sure the center bolt is as tight as you can get it, and the outer one is a little loose so that it will spin freely. Put the center bolt into the drill, making sure you tighten it in real good. Now take your dummy, whether it be a stuffed, or blow-up one, and cut a piece of the 1x1 wood/square plastic molding, so that it will fit the length of the dummy's back. Drill a few holes in the wood/plastic to slip some string through (leaving 1 hole at the top to be attached to the motion-arm. See figure #2) attaching the wood/plastic to the dummy's back like a backbone. Sit the dummy in the chair and measure from the top of the backbone to the outer bolt on the wooden circle (making sure the bolt is sitting at its lowest point to the floor). Cut another piece of the wood/plastic to that size, drill a hole in the top and bottom, and attach it to the dummy, with the long skinny bolt and the other end to the circle. On the chair I added screws and some string so that the motion arm and the back bone can't move side to side (see figure #3). To make the skull cap take the soup can and drill a hole in the top. Then drill a hole in the top of the metal bowl, attach the can to the bottom of the bowl with some string, leaving it a little loose so that when shaken it makes some noise (see figure #4). Hook the skull cap to the dummy's head with some double sided tape. Then hook it to the chair by attaching a 2x2 to the back of the chair so that it is about 12" higher than the dummy's head, and hook the skull cap to the 2x2 with some chain (see figure #5). Lastly, you can customize the chair with wires/strobe lightsetc. to add to the effect.