Best advice is to just go with whatever one GIVI recommends for that box, they are type fit and fitting involves drilling holes in the box. So it's a permanent modification whatever you do and therefore something you want to get right first time.
If there's complaints about it not fitting correctly / sticking out too far then likelihood is that some of those might not have been fitted correctly.
I have attempted and got it wrong once before on a previous bike. Luckily this time, as the GT version, my V's backrest was fitted by the dealer. Even he admits it's one of those jobs they loathe as it's tricky to get right.
Couple that with the legendary Italian manufacturing
best just check the website for the recommended one and take your time fitting it correctly.