I would go with the steel tube and pack it diagonally in a suitcase or cardboard box with loads of packing.

Get a length of steel pipe with threaded ends and buy a screw on cap for each end.

Any odd shape is likely to confuse baggage handling equipment which is designed mainly for suitcases.

Baggage handling conveyor design presents several serious problems to the designer. Image a small airport with just twenty gates. Your bag arrives at one gate and has to be transferred to another. This involves being shipped to a central conveyor matrix which has to change the bag to the conveyor going to the other gate.

To do this they have a device, euphemistically called a pusher, which hits the bag with a heavy chunk of steel and sends it flying onto the next conveyor. This has to happen very quickly because it has to get out of the way of the next bag coming along the conveyor.