I may have found an easy solution that doesn't require 6" pipes or shaving some of the drum flange.
The diameter of the top drum flange being 4"1/4, it is less than the OD of a 4" PVC pipe, i.e. less than the ID of the 4" pipe coupler
My plan is therefore to add such a coupler at the end of my pipe, engage the mast as far as it will go (i.e. all the way to the drum and add a short piece of pipe at the end that will fit in the coupler. I'll add a pin to make sure it doesn't fall off while traveling. This remains at the planning stage at this time, but I'll try a remember to add pictures once I complete the storage.