I’ve got a Thule Atlantis 780 which, oddly, is about 480 litres. It came with those pincer things spud mentions, although I ended up buying some special bolts for about £20 that slide into the aero bars. That leaves more space inside the box, and holds it lower/closer onto the bars and looks a bit neater. Since all our old luggage was knackered, I also got a set of 5 Thule bags that fit perfectly inside it, so packing it is a doddle now. I got it all from roofbox.co.uk who were very good. They even rang me to query something on my order that didn’t make sense (although it did, but wasn’t obvious).