Its Common for Time Trial bikes to put bar end shifters in the front end of the aero bars.
so since those like Pauls Thumbies require a set of bar end shifters to be purchased first,

try that before buying the thumbie mounts..

I have a profile for speed old aero bar set, It's 1 aluminum tube,
bent to be bullhorns and aero bars,
in a bin around here now,
they offered a bracket to mount downtube levers,up front,
right at hand, so while hammering the course, the racer could change gears
without coming out of the aero crouch.