As I’m giving in to getting a rack, thought I’d revisit “luggage solutions”... seems to be 3 main approaches, possibly combined...
1. The fixed box ... presumably drilled and u-clamped... I like the look of https://www.zarges.com/uk/products/boxes/zarges-box/
2. The removable suitcase ... although waterproofing likely an issue especially for vintage cardboard cases!
3. The soft waterproof bags from the sailing and biking communities. eg https://www.ortlieb.com/uk/products/dry-bags

Also mentioned
4. The boot bag - almost an allin1 rack bag! https://www.boot-bag.com/morgan.htm
5. Motorbike mounts and top box eg https://www.givi.co.uk/givi-products/cases eg see http://www.talkmorgan.com/ubbthreads.php/topics/318904/Re:_An_Alternative_Luggage_Sol

Any further thoughts?

M3W5sp 2015, MSCC, MTWC, Oxon UK