What size Bike Barn or similar to fit XJ6 Diversion F with top box?

I'm fortunate to be ordering an XJ6 Diversion F next week, fitted with a 39L top box.

Given that I commute all year round I wanted to invest in something more than a throw over cover and was looking at Bike Barn or Bike Home(?); the fabric covered metal framed, drive in pull over ones.

Unfortunately I can't tell from the pictures on the adverts which size I'd need for the cover to clear the top box at the back.

Does anyone out there already have the same bike, with a top box (39L or more) and uses such a cover?

If so please would you recommend the make and let me know what size to order?

Alternativeloy has anybody seen any of these at retail (in London or neighbouring counties) since it would be much easier to ride up and check it fitted instead of purchasing off the net?

Thanks for your help

Regards

Steffan