We must confess that the thought of putting any seat-mounted bottle carrier on our bike fills us with revulsion; it makes any bike look like a golf cart. But since it is Arundel making this Tri Clamp Kit, we'll give it a second thought. Maybe some day we'll do an ultra-marathon. We like the water bottle cage designs and Arundel's sense of humor. Check out Arundel's Monday in Hell video for a laugh or two.
We guess that our feeling is that you shouldn't use any of these devices, but if you must, and some of you absolutely must, you owe it to yourself to strongly consider the Tri Clamp Kit. It does a number of things we like. It is very simple; a block of nylon-reinforced plastic with bosses and holes and a simple coated-steel band clamp (coats so as not to scratch your seat post) that is tightened via a single brass Allen bolt at the back of the contraption. The mount comes with two different-sized bands and should work on just about any round- or aero-tubed seat post or integrated seat mast. One of the few it won't work with is the integrated-mast Kuotas; we were surprised to find it will work on the latest iteration Trek Madones.
Just try both band clamps on your post. The clamp that fits better is the one you use. The design has the bolt tighten the clamp in the same way that your brake levers are secured to your handlebars. Once the clamp is tightened in place, you can put one or two bottle cages on the back. The cages mount on nickel-plated brass inserts (to prevent corrosion) molded into the nylon. Arundel suggests going with the grippiest cages you own, like its Mandible. Not so much like the Sideloader. Whether or not you can notice from the picture, the cages aren't held perpendicular to the ground. Arundel found that if the bottles are perfectly vertical, riding over bumps loads the cages like a catapult and hurls bottles.
In addition to holding two bottles, the Tri Clamp Kit can also hold either two CO2 cartridges or one CO2 and one lightweight CO2 chuck. To get the chuck on, you need to screw in the included stud/locknut into one of the threaded holes. Just to show their design chops, Arundel has center-drilled the little stud so the CO2 chuck valve won't be damaged if it's over-tightened. The claimed weight of the Arundel Tri Clamp Kit is around 160g.