pretty simple: slot a tube lengthways much like they do in the frames, say an inch long ,
to make it compressible.
stop drill at the slot bottom round, to resist further splitting

get a seat tube binder clamp band .. that fits over the tube end to squeeze it..

If ID is not a typical seat post size, like 27.2 or 25.4, there are machined shim sleeves to make it so..




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