You could always make a pair, too. Start with a stainless steel bolt with the proper threading. Drill and counterbore the shaft to allow the cable to pass through but restrain the housing.

You can spin the bolt and file/grind/sand the bolt head into the truncated conical profile Campagnolo uses.

You have to flatten the sides but a file would make quick, clean work of that.

You could use a pair of jam nuts as the adjusting feature.

With a stainless steel bolt, no metal finishing is necessary and it won't rust.

Yep, sounds very do-able. Hand tools are all that's necessary except that a drill press is probably necessary to drill and counterbore the deep, straight hole.

Assuming you already own a drill press, the cost would be minimal (buying two bolts). If you had a good attitude, it could be fun to do.