Need some assistance.

I don't, it's just placed on as you would for a pipe, so far it's stayed on. Once the loads attached it's no going to move.
I did once for a trip to Cornwall use a few zip ties, so that would be my permanent way to fix.

 
In the picture you can see the pipe lagging around my extinguisher holder
held in place with velcro works a treat and easily undone 

Ok so thanks for all the assistance. I have gone for the pipe lagging as it is cheap. Turns out it's also very effective at cancelling the huge amount of wind noise that developed after I took the box off the top. It's now silent at all speeds and a bonus it's not awful to look at and it's easily replaced if damaged. Winner winner chicken dinner!

It's possible I need to be taken to one side and shot. This roof system is developing mission creep. I was following a gorgeous old 911 yesterday and it had a roof rack on made with wooden slats. I want to replicate the look. What is a good species of wood to use and how many slats should I look at with an eye on keeping the weight down but still being usable?

Any wood suitable for outdoor applications, would be fine, think teak, oak, Iroko, Cedar. Trouble is they are all heavy. Cypress is generally lighter, but not sure where you will get it from. Teak is probably too expensive, so Iroko would be a good alternative.

Or, go for a composite (plastic), that would be strong, light, no maintenance and will look good from a distance.

Just looking up dry weight of wood and oak is about 730kg/m3 while iroko is only 660kg/m3 so a suitable iroko slat would be 890g so with the bars and 6 slats over the meter width of the roof I would be less than 8kg total. Not awful.

That is interesting about Iroko.

I just thought I would look up Cypress, it's 510kg/m3, so a little bit of a saving, but Red Cedar is only 380kg/m3 and that would be a really nice wood with a wax/oil treatment.

www.simetric.co.uk/si_wood.htm

Trouble is Cedar wouldn't be as hard wearing as Iroko, so that might rule it out.