Fivemega makes this
Mag-D socket. I haven't really seen anything for the Mag-C. The brass ring looks like it could be turned down with relative ease if you know a machinist. You could do a ma_sha1 and chuck it in a drill while you file it down. The socket screw holes would probably be large enough to mount a small mandrel if you don't push too hard.
The best purchase option I can think of is a combination of Download's
MagCTower and fivemega's ceramic
G4 socket. Fivemega's not selling the G4's at the moment but probably will again at some point. Perhaps a want to buy over in B/S/T? Also the 4 mm pin bulbs are a little small depending on what you had in mind. He sells
5.3 mm sockets for an MR16 if you like that option.
JimmyM had a
feeler thread for Kiu replacement sockets. There's some info on the fivemega sockets. Perhaps you could post in that thread to express an interest or ask about the project's status.
Edit: That was a stupid idea. Estimates from the picture are by the time you reduce the diameter of the fivemega brass disk by ≈8 mm, the wall thickness of the 'spring' part of the disk will only be about 0.5mm thick. Perhaps you could just mount a bare socket onto something like a 1" copper end cap you cut slots into for tension. JimmyM's thread I believe mentioned a source for just the socket.