Les
Two points - Firstly Calton cases do a D ring option for their hard cases , so careful drilling and positioning is NOT a problem
Then I have the same problem eith guitar bamjo and mandolin ! I did
an adaptaton to a shopping trolly (two wheels and throw away the bag)
A length of welding rod and six inches of half inch copper tubing made a handle that came from the original handle up between the head ends of the two cases and a couple of elastics with hooks on to stip[ em falling off . Even stairs were not a problem ,except for the weight of two instruments in calton cases !