You could convert to clamp on downtube shifters pretty easily. The only caveat is that if there is no "bump" on the dowtube to hold the shifters in place, they might slip. then again, maybe not....

The other option would be barcon's.

All you'll need for the downtube conversion is a set of shifters, which are a dime a dozen. Heck someone here probably has a set that you can have for postage. You'll need to mount the shifters, re-run the cables, and adjust. Very easy.

Barcons aren't to hard to install either. But barcon shifters are more expensive, and getting more so by the day.