I haven’t had any problem with the seat post, but I kept getting the slipping problem with the expander cap in the fork steerer, the guy at the bike shop suggested to add some superglue, one drop on “plates” of the expander and just let it set after I retighten everything, works like a charm ever since.
Now I do admit glueing your seat post is a little out there, but perhaps you could use some thing like an “expander” inside the seat post, maybe some polystyrene or the like, just to act as a brace against compaction from the clamp, similar to how the stem and expander work on the front.
Hope it gives you some inspiration.