These folks sell the d springs that fits a d shaft that you could sleeve over the shaft.

http://www.lectra-fasteners.com/


If you wanted to go the shim route, I've seen this done for old radios so it may work for the Mk2 but I have not tried it.

Get a pair of tin snips, 2 pair needle nose pliers, and a few large binder clips. Not quite spring steel but close. Use the snips to cut a 1/4" wide by 1" piece of binder clip. Holding the metal in one set of pliers, use the other pair to wind the metal around the pliers. You're looking for a "G" shape. You'll have to experiment with the size of the D in the center. Trim the outer ring to size. Then squeeze your d spring into the knob and slide the knob on the shaft. If you want to 'barb' the tail of the G, it should help hold when rotating.