We have a boat bag for long trips. The model we have has been superseded by this one.
https://www.over-board.co.uk/collec...waterproof-backpack-dry-tube-60ltr-blackWe also have the Morgan plastic case with cover, and matching mohair boot bag. The case is so big and unwieldy that it's a chore to use although it looks good on the rack. The cover is a poor fit around the bottom and even though they came as a package its almost as if the cover is for a case without wheels. They all do that sir. The boot bag however is just the right size for all our hiking gear so gets used on every holiday.
Last year for our Scottish tour we decided to get most things behind the seats. We used the MMC leather holdall, plus various soft bags. Only the boot bag was on the rack. I fit with the zip downwards, but in wet weather it benefits from a bin liner inside.
