Put 'roof bag' into an Amazon search and you'll get plenty to choose from. Essentially it's a waterproof canvas bag which sits on the car roof between roof bars and you can stuff it full of family holiday stuff
We used ours for camping holidays and when it wasn't in use it got folded up and chucked in the loft. Simples, easy to fit and use and felt secure on the car roof.
I can't remember what brand I bought but I guess they're much the same.