I also have a real fear of roof boxes, having seen one fly open on a car that was about five or six ahead of me on the m5..
I had a roof box come open on me on a French Motorway. I'm not convinced I closed the front clip properly - I think the wind got under the edge and ripped the box open at the hinges - only the locks kept the lid on. Luckily most of the gear was strapped down inside and only a blanket and a sleeping bag flew out. Also lucky it was a quiet french motorway and not a busy UK one so was able to recover the items. To continue I used the internal luggage straps around the outside.
Bought a new box for the next year and now always put a couple of straps around the outside.
As a side - why are plastic roof boxes so expensive ? Surely, they can't be that expensive to make once the mould is made ?
First time we used our roofbox it came open on the A1 on a Sunday afternoon - the inner tents of our brand new tent were in it... arghhh...
Luckily the A1 was very quiet, the inner tents landed on the verge and were 'saved', DD's underwear was a goner.
The police came and closed the A1 and let us retrieve our sleeping bags from the hard shoulder.
We were just really lucky we didn't cause an accident and always double check the roofbox before moving off - one of the reasons for trying to find a car where we won't need one.
I'm just wondering how well I've loaded our trailer.
Boot generally isn't too bad - bedding, clothes, kids toys and food, all in bags/boxes (and all boxes below seat height).
Kids have 1 box of misc stuff between them, with a bungee cord round it.
Generally a few things in the footwells (the DW seems to always end up with the water container in hers), but nothing _too_ bad...
We have a roofbox (only tend to use it for clothes these days). I was paranoid about loading it the 1st couple of times and made sure the weight was well under the 50kg, and it was well strapped down inside. Use it alot less now (in fact haven't put the bars on this year - yet), but still check it LOTS!