We use an old leather suitcase that I found on E-Bay - for period effect and style. We have a 90's 4-seater with a standard stainless steel rack on the back that holds the case like it is on a shelf. I have lowered the rack about 85mm by getting replacement rear stays made (Heart of England)so that with the hood up I can still see over it in the rear view mirror. We drive the motorway from Bilbao to Biarritz and beyond a few times each year, so I feel a lot safer if I can see the Spanish and Portuguese lorries following me.
The case itself does get wet, but it looks right. We pack everything inside it in a big strong plastic bag and we have had no problems at all. That leaves the interior space in the car for wine and other comestibles.
A different solution, probably peculiar to our type of 4-seater rack, but it works for us.
Yes our solution too. Bought a big old British black leather suitcase with aluminium clasps for 20 euros at the second hand store down the street. Also comes with interesting stickers from previous owners. Will put our kit inside a plastic bag and stow it in the suitcase. Or just use the suitcase for side screens and waterproof car cover and put our kit behind the seats. It looks the 60's part and it costs, well, 20 euros.