I have done a bit of research on this but never actually bought the Paul's thumbies so take this with a grain of salt. It seems there are a couple of Paul's thumbies mounts. You would definitely want the ones with the same boss as is on a downtube, I think it's the shimano model. You can then get the shifter spacer/washers for aluminum frames(not radiussed). You can get them at Riv bikes if you want:Shimano Bar end Shifter Spacer/washer (Pair) - 17135. After that, it seems you can just bolt your shifters on and be all set. Hope that helps get you on the right path at least, if I'm wrong on any of this, sorry. Good luck and post your results.

Edit: you could also grind flat a radiussed pair of shifter washers if you want. I did for an aluminum bike and it worked great.