Brian- Great timing! I'm about to ship a "rack" to a customer. Actually a Seat Post Thing with a back of bag support strut. For those who are too young to know what a SPT is it's a stem/bar like attachment to the seat post so a handle bar bag can be mounted behind the seat. The support struts are to keep the bag from sagging onto the rear tire if heavily loaded. The STP is made of .049" wall for the extension and bar stub, the post clamp is .065' only to get the ID needed (31.6mm). If my lathe was running I might have turned this down to .049" or so but at only a couple of inches long the weight is not too much anyways. The struts are 3/8"x.035. Thick enough so there won't be much worry about denting and with the mostly vertical loads bending shouldn't be an issue. This project was interesting. I have made a number of SPTs but always had the bag either small enough to not touch the rear wheel or supported by a traditional rack. The support struts were a fun job which went through a few designs and during the fabrication had some detail changes. Here's a set of photos. Andy.