Transporting my Board

Hi there,
I know this will sound a bit naff, but could anyone offer me some advise on getting my surfboard down the M5 on a car with a roof box!!!
Normally I use my wraprax with no problems, but this time, I'm taking the whole family including the dog and there's no room for the wife's extra's unless we take the roof box!!! What is the best way to get the board down with the roof box on?
I've had a go at putting the wraprax on the box, but it doesn't look that safe, so any ideas would be appreciated
Cheers
I know this will sound a bit naff, but could anyone offer me some advise on getting my surfboard down the M5 on a car with a roof box!!!
Normally I use my wraprax with no problems, but this time, I'm taking the whole family including the dog and there's no room for the wife's extra's unless we take the roof box!!! What is the best way to get the board down with the roof box on?
I've had a go at putting the wraprax on the box, but it doesn't look that safe, so any ideas would be appreciated
Cheers