Hey mate
Short answer is you can't.
There is an auto electrical mob in Perth which does an install where you're looking to fit it but when I contacted early last year they wouldn't sell me just the tray. They were happy to sell me everything for about $1,500 but I'd still have to fit it all.
In the end I ended up making one & doing it myself.
I used the same location in my car (2016 SDV6 HSE) as I also had something behind the right side panel.
My writeup is on here if it helps.
Link if it works
2016 L494 SDV6 HSE - Auxiliary Battery Tray - Fabrication & installation
Cheers
Craig
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