Profile Design's HC Mount is a nifty way to carry enough fluid with you during a long time trial or triathlon. It consists of two aluminium crossbars that you attach above or below your aero bar extensions with zip ties, to which you then bolt a Profile Kage (the mount is also available without this).
The crossbars are long enough to span extensions with a width separation of 130mm. We found it fairly easy to use, and with practice we could pull the bottle out of the cage and take a drink while still on the aerobars. The Kage is quite snug so you won’t lose your bottle if you hit a bump.
One tip we’d recommend before mounting: wrap small pieces of rubber around your extensions where the zip ties go. This should prevent slippage and scratching of your bars. Also – and Profile do warn of this – we wouldn’t advise using it all the time as it'll place extra stress on your bars.