Ive been toying with the idea of making one myself. My first thought was a large chunk of white styrofoam... sand and shape to fit, then fiberglass over it, then use a solvent to dissolve the styrofoam. Im not a real big fan of fiberglassing things and the fact that im an old sheetmetal guy that used to beat on cars... leads me to believe I can make one out of sheetmetal.
The plan... Build a form buck out of lumber then form the sheetmetal over the top. Forming a right and left half, and then joining the two together. All in all it shouldnt be too hard, I would probably have to do some cutting and welding of the corners to get it all worked out since im not setup to do shrinking.