It sounds like the aligned slots of the clamp and seat tube are allowing the corners to dig in at the back of the post (where the maximum compression stress is anyway). This used to be a significan problem with thin carbon posts, and would often lead to post failure.

Try reversing the clamp so the clamp's slot is in front, while the frames, is in back. The non slotted part of the clamp and frame tube then support each other across their respective slots maintaining a uniform compression without corners digging in.

To make this work the clamp has to be greased well on the inside (outside of the seat tube) so the clamp and tube can slide over each other as they're tightened onto the post. This will give you a more uniform hold and may also solve the creaking, but at the very least will reduce the chances of buckling the post because of local stress.

One other note --- the corners digging in is often an indicator of a slightly loose (within the tolerance of a correct post) fit, and that may be the cause of your creaking. Ideally a post should be a snug running fit within the frame, such that you may need to twist slightly to move it up or down. If yours is easy to slip up and down, the fit is a bit loose. Sometimes you can solve this with a different brand post made closer to the high limit of the working tolerance.