OK, this is a shaped CF post in a CF frame, so many of the classic, proven methods aren't available to you. Plus there doesn't seem to be a collar so I gather it's some kind of wedge in the back.
If I saw the cluster and clamping system, I might be able to come up with something, but others here probably already did that, so I'll give you a last ditch, when all else has failed, off the wall possibility.
Find a length of PVC pipe with the largest diameter that will fit down the seat tube. Cut it off longer than the seat tube and drop it down and see if it finds some kind of solid bottom.
Now, improvise a retrieval system by drilling a hole and setting a cross pin near one end, which will be the top, and which you'll snag with an improvised or bought crochet hook. Now, you'll do all your cutting from the other end, and the goal is to cut it to a length such that the post is supported on it at the right height. Hint - start long and do a rough cut, then come to the right length by degrees, being careful not to cut too short and have to start again.
When you're finished, you'll still clamp the normal way, but the post will act as a dead stop and provide that added bit of resistance to slippage.
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FB
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