E-Z Lok Threaded Insert, Zinc, Hex-Flanged, #10-32 Internal Threads, 13mm Length (Pack of 50)
https://www.amazon.com/dp/B002WC8TUC/?tag=caraudiocom-20
#10-32 X 2" Stainless Phillips Truss Head Machine Screw, (25 pc), Fine Thread, 18-8 (304) Stainless Steel, by Bolt Dropper
https://www.amazon.com/dp/B07D1KF92M/?tag=caraudiocom-20
And you can get different length screws of course... just make sure they’re the same thread size and count as the insert. I also used locking washers to keep them tight.
It’s a Caddy trunk so it’s huge. The seat is tilted so it’s probably 8-10” from the bottom of the back seat and 3-5” from the top of the back seat... so there’s still room behind the box.