Go to a Play it again Sports and buy a used football helmet. GO to Home Depot and buy a threaded floor flange, along with some nuts and bolts which fit the holes on the flange, and a piece of pipe that is pre-threaded to fit the flange. I went with 1 5/8" pipe(IIRC), which is the hole size on standard(not olympic) plates. You could probably buy a pipe to fit Olympic plates though.
Using the flange as a template, mark spots on the top of the helmet where you will drill through(remove all the padding first, it usually either snaps out or is velcroed. Put the helmet on, and place the flange so with your head in neutral(regular standing posture-chin level), the pipe will come out a few degrees below straight up(leaning forward). Drill your holes, bolt on your flange, and screw the pipe back in. Put your plates on(you will start very light with this, the weight on top of your head makes for some very interesting leverage issues), and secure them before you start working. I use a couple wraps of duct tape, since I could find collars that would keep the weight from sliding.
To use, get on all fours to do rear raises; lie on your back with your head hanging off something for front; lie on your sides for side raises. You can also try different variations, but I never have, I just use it like a four way.