I've got an idea that just may be crazy enough to work:

I've got some bullhorns laying around that I'd love to throw on my flat-bar for a more narrow profile and more hand positions. But my bike suffers from grip shifter syndrome. The answer? Find some nubby bar ends that are narrow enough to hold my grip shifters, just on either end of the stem clamp.

Something like this:

I've been scouring the internet, but it's tough to find any information about the actual diameter of these, as the primary use is hand gripping.