Have you seen or considered the cantilever brake pad stud configuration? It's at least cheaper than the cast aluminum adapters.
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I just tried one in my C Model fender hole and the stud is too big, would need to be drilled up a mm or so. It does work in this post war frame though.
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I've always liked the idea of having a Pitbull caliper or roller cam style center pull brake mounted below, but haven't built one up yet. I like my drums so I don't have to worry about wearing finish off the rims.