Thanks Ian, when I was first thinking about doing it I asked for suggestions on a couple of other forums and the consensus was that I was daft to try.
I have also modified Nikon Photomic prisms by removing all the electrics, cutting down the chassis and making a new top to produce my own version of a standard prism for a cost of 20 instead of 120 that they sell for. As you can see they are not as pretty as the Nikon original but they do the job.