And then mount it where? Stem? Somewhere on the bar? The problem with my current setup is that the stem shifters fit around the threaded to threadless adapter ok, but the adapter widens with a lip below the stem goes on:

and it's this lip and the stem that the shifter levers can't get over. If I have the adapter set up high enough and the shifters right above the headset, they can go underneath the adapter (but just barely). When I lower the adapter, they can no longer do that. If there's a good spot to put those thumb shifters to get around the problem, I'd really be interested as I'm sort of used to them as that's what I had on my last bike (a late 80's mountain bike - schwinn woodlands from when schwinns were still a bike shop brand and mountain bikes were only 5 speeds). Are there clamp on downtube shifters that'd be a better option? Don't know if they make these or where I'd find them (probably e-bay vintage ones if any exist), but that'd be an option I'm open to as well.