Originally Posted by McDave
I was going to suggest an add-on
seat post collar, but I don't think there's enough of the seat tube sticking up above the frame for the collar to grab on to.
Older road bikes usually had built in clamps. I wonder if it would be possible to cut the other side off also, file it down and put an oversized collar on it such as the one you have mentioned - that would most likely be the cheapest option.