I've made one myself, but only because I had a quill gaspipe post already.
Since I already had the bottom wedge nut and the uber-long bolt, I only had to drill down through the seat clamp assembly.
It took 3 drill bit and a trip to my neighbor's drill press, but it worked fine.

By the way, they look great on those Schwinns. Clean, clean.
I've often wondered if they could be made for the Cannondales, too.