Bontrager Seatpost Rack
Bontrager's Seatpost Rack is a great choice for frames without built-in eyelets, this versatile rear rack quickly attaches to your seatpost with an easy to use four-bolt design. Compatibility with Trek's Interchange System means it mates seamlessly with a variety of Interchange bags (sold separately) that install fast and secure. It fits both 27.2mm and 31.6mm seatposts.
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The rack has a nice finish and holds the Interchange trunk bag well. My only complaint would be that the rear light bracket broke due to vibration. I would recommend a rear light with a P-bracket on one of the seat stays.
I use this as an all around rack. In hindsight I would opt for a permanent mount rack. It would be excellent as a Quick release rack such as for a mountain bike used for occasional city use (which is it's intended purpose.) It limits seat travel adjustment, at least on my bike, and could interfere with rear tire clearance.