I made one out of a bike seat stem and some steel from a bike pannier rack. It looks kind of like the Barnett "Black Widow."
I just stuck a handle bar grip over the seat post, and made a wrist stabiliser out of more of the rack, with some webbing to make it comfy. I used speargun elastic and part of an old leather belt for the business parts.
Only problem is that the "prongs" are separate bits of steel, so no matter how hard I tighten the seat clamp on them, they eventually rotate with use, and it ends up cutting the elastic somehow. I need to bend a single piece of steel for it so that stops happening. Otherwise it works great!
PS the only reason I did this is because I can't buy one. They are illegal here. /ubbthreads/images/graemlins/frown.gif