This seems to be the main topic of discussion today. That CTC link keeps appearing everywhere!
I reckon there will be a J-Tek Shiftmate that will do the job, but there are other alternatives. One option is to machine your own narrower spacers. The other option would be to buy some wide steel washers, and experiment with clamping the cable at different locations from the conventional clamp spot à la the Hubbub modification. This effectively changes the shift ratio. You may be lucky and hit upon a "sweet spot". If you do, mark it carefully, and use a hacksaw to cut a groove in the washer so that the cable will always sit in the ideal spot.
Also, you can eliminate some of the sloppiness in the rear derailleur by replacing the jockey wheels with modern ones. There may still be some slop in the spring though.
If the derailleur is in good condition, a solution can probably be found.