You can flatten the "half moon" shape on the back of the backing plates with a file or a piece of coarse sandpaper laid flat or, best of all, a belt sander. That's how Chris Kelly makes them. The real backing plates work best and modifying them is pretty easy.
I put a set of Kelly Take-offs on my rain bike using Shimano 7-speed indexed dt shifters and they are the next best thing to STI/Ergo.