I've been trying to decide how I'll build my own. I may see if I can get a round cylinder to fit my lens perhaps, with tabs on either/both sides. A band around the back of the camera would be pretty useful.
Maybe this weekend I can try something out for my own camera that works well.
I really think it needs some type of shield to block the MH from blasting the light meter sensor that is above and behind the lens itself. So it really sees what the lens sees directly ahead, rather than bathing in 350w, 500w, 800w or more lighting from around the camera.