Take 2, 1/4 inch steel rounds and sandwitch 10 or so brass plates between them. Bolt everything together with that 1/4 center hole. The steel rounds will need to have your hole pattern already drilled in them. The rounds should also be larger thanthe brass and shold have 4 equally spaced holes spread out larger than the brass for extra clamping bolts if needed. Then just drill through the existing holes in the steel, hell this is the same kind of jig i would build for doing this manually.