Just closing the loop on this one. I tried cutting out a piece of aluminum from a piece of downspout I had around the garage. It fit tightly between the handlebar and stem but it must have been too smooth because I could, with a little effort, rotate the handlebars in the clamp. I ended up ordering a 24.4mm -> 26mm shim from Velo Orange and I put it in today. Works like a champ. The surface of the shim is micro textured so it provides some grip in the clamp. After installing the shifters, running the cables, and wrapping the bars I should be good to go.