There need to be a inch + of seat tube squared off , above the top tube
to use a separate seat post clamp.. 6~mm is insufficient
what other solutions are there?
major frame modification, you might braze another tube in
with a 1" or 7/8" inch ID, fit it with a pinch bolt ear
then you can use a 25,4mm or BMX 7/8: straight tube seat post,
and fit a separate saddle clamp on to of it