Bike Friday uses regular commercial suitcases. Initially they used a Samsonite Oyster, then switched to a slightly larger Carlton (which I have), and now use a Samsonite Flite. The only difference is that they leave off the usual inner fabric layer which also includes some little pockets. No need for these and they'd just get torn up by the packed bike. They also include some spacers so if there's any pressure applied to the outside of the case it will be resisted by these instead of possibly damaging bike components.

One caveat is that mine came without any reflectorization and I was getting a little nervous on my first trip when I was riding back into London after dark and realized that while I had lights on my bike the trailer/suitcase behind me might be hard to see. I added reflective tape to the suitcase the next day and that also makes it easier to spot when it comes around the luggage carousel in the airport.

The trailer frame has been quite handy as a utility trailer in addition to carrying the suitcase. I cut a sheet of plywood to fit on top of the frame, drilled 3 holes to attach the frame, and some additional holes along the sides for ropes/straps/bungees. It's been used for a variety of tasks - Costco bulk shopping trips, moving some furniture, carrying my kayak, etc.