Weve an Odyssey roof box (cheap thule) and its never leaked. If you think about it 70 down the motorway with spray is regular so I wouldnt worry about it. We put our bedding in when we tented as it squashed quite easily!
Our Thule Pacific 200 kept the bedding and other bits and bobs dry, even with a drive from Cumbria to Cornwall in the rain.
But we did notice when we first packed the box, that with two people pulling it shut it can be locked with the front not fully down, so a rearrangement and re-close it was perfect and looked the same as when its closed and empty.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.