I think you'd struggle to get a decent sized Givi box in the pillion area in addition to the standard topbox on the back but start off by taking a few measurements - no point in looking for mounting solutions if there isn't space for the box itself. See if you can find/borrow a Givi box and just place it on the pillion seat - then see if you can sit on the bike!
It would also be a right bugger to try to open the Givi with the opening mechanism jammed up against the back of the metal box. Move it further forward to give clearance and it would encroach on the rider space. I've got my Vario case mounted on the pillion area and I had to remove the backrest from it to give myself more room, otherwise I couldn't sit up straight when I stopped. There's not all that much space.