Now I don’t know much about suitcases except they need to be cheap and expendable.
May I expand a bit? Audrey and I have used Globetrotters for more than 30 years and more than 450,000 Morgan kms. They certainly are NOT "cheap and expendable".

We bought our first for 175 quid but their price skyrocketed after the British owners were bought over by Japanese interests, about 20 years ago. Harrods distributed them in London but Globetrotter grew immensely and has one of their Flagship stores at 51 Piccadilly.
Yes, budget 2000+ pounds for each. But I speak from a situation where my wife and I spent 4-5 months mogging every year on 2-3 continents. With that type of Morgan time, any cost for precisely the right luggage is justified. They look period, come in any colour one could want and they are completely rain-poof at any speed.
https://www.globe-trotter.com/collections/centenary They are infinitely more convenient than any other option available at any price. As you know, they are made of a leather-looking polymer, with canvas lining, chromed fittings and leather corners.
We learned that it is wise to choose a Globetrotter that suits a trad. We soon grew out of the
two most common mogging luggage problems 1. Packing the rear luggage rack, cantilevered past the axle, with too much weight. That very much ruins trad balance, prejudicing the cars handling at a time when you want to enjoy it most. We slowly learned how to pack and where to put anything weighty (over the axle in the compartment behind the seats)
2. Buying a Globetrotter
or anything, that blocks your rear view mirror.
We settled on their thirty inch model. We packed heavy stuff, (shoes toiletries) in two Gladstone bags (which are shaped for the axle compartment) with a laptop in between We learned to get them without wheels and forwent the leather straps as we all use leather straps to secure our luggage to the rear rack anyway. Two sets of straps look odd. They do come with locks. Sorry, but don't buy them, if at all possible, with the pulling arm. Two (little) wheels but no arm. It obstructs a proper strapping on the rack. However, it has been years since we bought one. The wheels look bigger now. (pity) Of course, you can later fit any size and type of wheel you want. We fit the luggage on the rack with the wheels down so they didn't show as much when on the rack.
Lorne
https://www.gomog.com/allmorgan/storagespace.html#LUGGAGEP.S. I have never understood the MMC hardcases or many others I see. Though they are properly sized and attractive, why buy a pretty hardcase that has to be covered for rain weather?!! Try to compromise aesthetics with practicality. That being said, chacun a son gout.
P.P.S.We began a fun hobby. There is not much trad space for shopping in a trad. So we would searched out luggage stickers at every stop and pasted them on. After a while they drew crowds! Especially ones from different continents. A New Orleans stickers were a big hit in Paris and London stickers were stars in Niagara Falls!