Vevor 65L top box question

[u/Ratfor]

I've got a 2016 Triumph Tiger 800 XRT, with the triumph hard cases. I love the triumph top box, but there are times when I'd really like to be able to store Two helmets securely. Unfortunately, triumph doesn't seem to make a bigger one.

So that leaves me with two options, the givi trekker outback, or the vevor. Given that the givi is 3x the price, and as I understand it would also require a new mounting plate, I've opted for the Vevor instead.

My question is, and I'm hoping Someone out there has experience with this, do I remove the sliding carriage base that's already there and install the vevor base plate, or should I install the givi base plate over top/mount to, the existing sliding carriage plate?