Hi there, going on a long road trip in May and wanted to check suggestions for a waterproof case or bag to take all our clothes etc? Are rooftop bags suitable or too big? Has anyone got experience with a large waterproof suitcase or similar? I have a 1997 Plus 8 with a standard luggage rack.

I have a hardly-used (because it's quite heavy) American Tourister / Samsonite hard shell case with stainless securing strap and mohair cover (the other stuff in the case is for illustration only).
It's big and bomb-proof, and no longer required as we don't tour any more. Available to collect in exchange for a suitable donation to charity.
PM me if interested. (You will need to log in and select "send private message").

We use a boat bag.There is a Mog specific one. Boatbag I can vouch they keep everything dry and don't block the 3rd brake light

Cheap solution is put everything in a bin liner inside a hard case.

Typo there I think…should be boot-bag.
I have used one of those for the past 11 years and it works well, with everything staying dry.I tend to put bulky stuff in it like shoes, washing gear, etc
In addition, I use a pair of “Hills Alive” cases that are tailor made to fit behind the seats. (sadly the company no longer exists)
The number of Morgan leather suitcases that you see second hand, tells its own story…look nice but simply much too heavy empty, let alone full!

My Overboard bag https://www.over-board.co.uk/collections/waterproof-duffels has kept my stuff dry on the back of an M3W and Roadster over many thousands of touring miles.

Can also recommend the Over-board bag - 10 days doing the NC500 in, part of the time, biblical downpours and, near Torridon, driving through a mini, near horizontal, waterfall shooting from one side of a mountain road to the other, everything in the bag stayed dry (the driver, however, did not stay as dry, particularly in the said waterfall which launched itself through the open driver's side sidescreen ....!) (its a Morgan, sir - they all do that!)

I can also vouch for the overboard waterproof duffel. It was well tested during a road trip through Wales. We also have 2 of the smaller bags that fit well behind the seats and being soft it easy to fit smaller items around them. Not having a Morgan logo makes the much more affordable as well!

^^^^ What a lovely picture

Thanks for all the rapid and useful responses. Will take a closer look over the next few days.

Hill alive bags are the business