Rimowa Strap On
A four-wheeled cabin case is close to perfect on smooth floors and useless everywhere else. Cobblestones, gravel, stairs, a platform with no lift — that is where it stops being a vehicle and becomes something you drag with one arm. The obvious fix is to carry it on your back, and the obvious problem is that a Rimowa has nowhere to attach anything to.
So the harness works with what the case already has: the handles, the shell, and the ability to wrap all the way around it. Nothing drilled, nothing glued, nothing that leaves a mark.

It is sewn from seatbelt webbing on an industrial machine, with a roll of hot pink for the strap that closes around the body — pink so it reads as deliberate rather than as a repair.
Every joint is a side-release buckle on a slider, so nothing is fixed to one piece of luggage: the straps let out and pull in to fit a different case, a bigger bag, or whatever jacket is underneath. It goes on and comes off in seconds and packs flat when the ground is smooth again.

The case is still a case: wheels down, handle up, unchanged. It just has a second mode now.