!
I have exactly that shifter setup on one of my bikes.
!

I took a mtb bar end, sawed off the curved portion you'd normally grip, leaving just a stub. Then installed the barcon shifter into it.

The trick is, that mtb bar end grips are not intended to have things plug into their tubes; thus often times the inner diameter will not be big enough. best odds are if you select old steel bar ends (thin wall tubes), aluminum ones tend to have thick walls unless they're expensive...



-background info on my usage:
flat bar, with 4 bar ends: 2 sawed off facing rearwards to mount the barcon shifters, 2 ski ended ones facing forward with TT reverse brake levers on them. Essentially double-sided pursuit bars so they can mount shifters in addition to the usual brake levers.