Mention was made of the OT Minoura double-cage mount for use in duplicating the classical bar-mounted ports-de-bidon. Be advised that the Minoura unit is probably NOT suitable for riding anything but paved roads - I had one that broke right off on me on a long dirt-road ramble. I suspect that fabricating an additional brace from the bottom of the mount to the quill of the stem would solve that, and it seems to me that many of the old units like this had that very setup.
Russ Fitzgerald
Greenwood, SC, USA