I know this is old. But if anyone sees the thread i wanted to confirm the stock mounting bracket is actually pretty strong. And there really shouldnt be an issue since you dont want to put much weight all the way back there anyway.
I had the givi on the stock bracket. Instructions that cane with the givi were easy. Spacers were a bit of a pain to work with.
I'm, not entirely clear about your exact set up.
Your Owner's Manual describes the standard rear carrier, under 'instrument & control functions' page 3-42 for my ES. This has a load rating of 5Kg. A few have attached a Givi universal rack to the carrier, just to take a storage box for use when parked. Others have removed the OE carrier & mounted a universal plate, with suitable spacers to the rear sub-frame. Always remember the max load for the bike is 205Kg (452lb).
Then there is the SRA2101 Givi alloy rack described, with a link to the fitting instructions, in post #2. This has a load rating of 6Kg & has a very neat appearance, but they do leave a pillion rider short on space, so many have re-engineered the mounts to move the plate backwards slightly.
Then comes the SR371, with a load rating sticker near the latch point of 10Kg ........
https://media.givi.it/XN-Prodotti/istruzioni/SR371.pdf
These are good racks, but again leave space tight if you carry a regular pillion.
One of the after market alternatives, that overcomes the issue for me, is the Altrider, with a 10Kg load rating. They don't come cheap though & a few early adopters had problems with cracking, said to be due to either over-loading or over-tightening the fixings; mine has been no problem though, but I do watch the load carefully.
I know all these load ratings may appear cautious, but heavy loads mounted behind the rear wheel spindle can lead to instability, so it's not just the pure strength of the plate, or the sub-frame, that has to be considered.