Older style A3, I’ve tried a standard Thule and it makes the car leak as the doors won’t seal properly around the feet.

Any other suggestions?

Maybe pay the £1200 audi want to fit rails

Are you sure the feet are in the right place? Have you tried moving them back / forward, in / out to see if the recommended distances are a bit out?

Other than that I can’t imagine anything else would’nt do the same thing unless you get roof rails or new door seals.