I have a bootbag that is a good option. A couple of extra poppers to hold down the flap over the zip as the zip is the weak spot for water ingress. I have gone to a suitcase with a cover as the zip is exposed and finding a case these days that clips closed with good seals is not easy. Last month we took them both and used the bootbag to carry reduced stuff across to the Hebrides, leaving the case at the hotel on the mainland as we stayed there again on the way back.
A solid case blocks off rear view so wing mirror driving takes over, the bootbag doesn't. I have a Cobra high brake light but unplug it and fit an led light onto the case with straps to maintain the extra light. Superseal connectors make this easy. The cases are lightweight as opposed to the very heavy Morgan one!
I picked up leather straps to secure the case and the brake light strap stops sideways slip as a bonus but not essential.

Plus Four MY23 Furka Rouge