Boot Bag and how to attach it

[Dickyboy88]

I have just purchased a V6 S convertible and wish to increase its luggage carrying capacity.
After some research, I have also purchased a 75 litre Vacation boot bag ( from the Boot Bag Company) and this sits on the top of the boot and is secured by straps under the boot-lid.
As it appear not easily possible to temporarily isolate the rear spoiler from lifting, I was wondering how other owners have managed to safely secure such luggage with a normally operating rear spoiler?
Any help much appreciated as I have a 600 mile trip to the Goodwood Revival coming up in 4 weeks.
Thanks
Richard