I put two of these together that I had lying around and bolted them on my seat tube, between the top tube and seatpost collar. I bent the two half-circle clamps, put them together and made a single clamp that fit perfectly.
Worked like a charm. A single P-clamp would have worked, but I didn't have one lying around.
I brought the two arms of the rack together with a larger bolt for the end of the clamped together loop so it wouldn't come off the clamps. I used these system for a good two years, with considerable weight. The rack stays were bolted to the drop-outs, but Giant didn't include braze-ons that year. The years before and after, but not that year, lol.