Not able to find the second rubber insert. But I do have a tidy garage now! 😀
I didn't actually use the strips at all. With a wider flush mount roof box there would likely be enough in a single strip to cut for either side of the box. Replacement strips are also available for less than £10 on eBay.
Good question Gareth. I have found one in my garage (unused and uncut), and the original printed instruction manual. I'll rummage in the garage some more this afternoon and post an update if I find the other.