I have taken my Brompton on many domestic and international flights by packing it in a suitcase. It has never been damaged. Here is my method.
I use an American Tourister Curio 2 Spinner 80cm suitcase. It’s dimensions are 80cm x 56cm x 32cm. It is a large suitcase, but the airlines have always treated it as regular sized checked baggage, and no extra fees have been charged.

My Brompton fits inside if I remove the seat and seatpost. I also remove the accessories and the hinge clamp brackets and bolts. They travel in a zip-lock bag. The hinge bolts would be close to the edge of a suitcase otherwise, and I want to ensure they do not get bent in transit.

The suitcase is semi-rigid, but maybe there is a chance that a big force applied to the large flat sides might damage the bike. To add some strength, I use a bracing post for extra support. It is made from parts from a plumbing store.

The post can be disassembled and reassembled as required. The colourful electrical tape at the ends of the post helps get a snug fit with the flat circular end caps.

The brace post fits through the bike. It adds a lot of strength to the case.

When I travel, I enclose the bike and the brace post inside an Ikea Dimpa storage bag. The bag ensures that any dirt, oil or grease doesn’t escape and affect my other items.

I pack all my other items in the space surrounding the bike. They act as extra padding, too.
