With any add on luggage carrier, I always check after 10 mins or so, as any driving will shake things down a bit. I drove to Cornwall and back with the boot bag in place and absolutely no movement. But you do need to tie up the loose strap ends to stop them flailing around on the paint - I used hair bands as there’s always a gazillion of them around our house!
Well John, you were right. I bought the boot-bag and just did a 2-3 hour journey on M-ways and country roads. Only say 1 hour of it was in rain and not really torrential; simply regular, well, rain. Like you I found a little wetness under the boot straps. Luckily i had remembered your advice and used a shopping bag and (ironically) the large, clear, plastic bag that came with the boot-bag placed between boot lid and contents. The bag itself worked fine. Thanks.
Perhaps it’s all torrential when you’re doing 70!
A Transit van, or similar! 
