In reply to climber34neil:
I made a pair out of some old mountain bike SPD shoes, and a pair of crampons I bought off here.
The shoes have a very rigid sole, with a plastic plate embedded where the pedal cleat attaches so they are really stiff.
The crampons were an old pair of Petzl fixed mono-points which were only £25. I chopped off the bits of crampon not needed with an angle grinder, they already had suitable holes to bolt to the shoe.
I cut the tread pattern off the sole of the shoe with a hacksaw as it was quite deep, to leave a flat surface to mount the crampon. Then I just marked where the holes needed to be, and drilled through the sole. By removing the laces I could open the shoe up enough to access the heads of the screws, I used flat headed pan screws and wide washers so they did not pull through the sole and to add to the stiffness. I used locking nuts on the outside.
Inside the shoe I covered the screws with tape and then replaced the insole and it is fine.
The only down side I have experienced is that the crampon point is quite short, they were very worn before I sharpened them. On a route like Sub Rosa at Masson where you toe into a roof crack this can be a disadvantage. I would recommend using a crampon that you can change points on.
Not bad for £25 though!
Andy