Boot bag good, I have used one. Just take a sheet of polythene and an old towel to put on top of the boot contents in case you get heavy rain en route. I learned this after 200 miles in stair rod rain. The water wicks along the straps that go round the boot lid. The towel absorbs the moisture and the plastic keeps the contents dry.
We did 10 days to the Italian lakes with boot only - but we didn’t take groceries! I did have my camera backpack, 2 carry-ons, and a couple of softs bags in an ND. We also did 8 nights on our NC500 trip with just the boot.
No possibility of taking hiking boots, and the Imelda Marcos memorial shoe collection had to stay at home too.
I actually bought the carbon fibre rack and suitcase for the NC500 and managed without using it, so I sold it on for what it cost me. New, they are too expensive really - £700+IIRC.