As witnessed by a number of TMers, we’ve always used a ‘clamshell’ Samsonite suitcase.
We then wrap it in a waterproof picnic blanket, then secure to the rear rack.
Lots of trips in the UK and to Europe over 18 years with this setup and no water ingress.
Case comes off at our destination and the blanket is then used for its original purpose.
4/4 - Stanley