Why so much hate for the undermount set up? Just curious
It seriously limits BTA hydration options.
On Alpha X, there are a pair of BTA bosses built into the stem cover that can be used when using the undermount setup.
I have the Alpha C (proprietary stem). I guess you applied some 'lessons learned' on the Alpha X. :)
Your problem is the proprietary stem, which the Alpha X wouldn't help you solve. The Sigma XF also has bosses for a BTA bottle that is perfect when rocking the undermount setup. But this doesn't help you solve the problem of where to keep your Garmin, which is a problem (to date) that Nick has failed to acknowledge exists.
You'll get no argument from me about lack of flexibility with the proprietary stem, wish I'd known more when I purchased.
That said I still don't understand the trend towards 'tall' frames in the market when it's so easy to pedestal the pads for a taller fit. Another strike against undermount extensions on a tall frame is the bullhorns being higher than one might like, plus the fact that it limits your choice in extension bars if you don't want a super-low hand position when on the aero bars.
There's just really nothing to like about undermounted extensions, they're a last resort IMHO and come with real limitations.