Does your bike have an integrated seatpost? Or can you remove it?

If you can remove it, you have lots of options. If you cannot, then things get limited.

I asked this question of a world-level elite, and she recommended Pika Packworks. I had a custom one made for my Tiemeyer (long seat mast). I had concerns about using it for sending a bike across the country for races and moves. It worked no problem. I highly recommend them. I even packed it up and shipped it FedEx for a cross-country move.

That case is now being used by a lady on the US Paralympic squad protecting her $10,000+ LOOK L96 for international travel. If that's not a good endorsement, not sure what is