Bike Friday supplies a couple 'crush protectors' to protect against the relatively soft plastic sides of the suitcase from being pushed in and damaging the bike. They consist of a couple circular pieces that screw onto the ends of a piece of PVC tubing:
https://store.bikefriday.com/product...roducts_id=652
You place one of the circular pieces attached to the tubing under the bike and then screw the other piece in place, adjusting the height to closely fit the closed suitcase lid.
I think the equivalent could also be made from items at a hardware store.