Here’s the box I use … old pic … it’s now on the floor flat under the side seats as see in this thread:
https://forum.ih8mud.com/threads/rear-toolbox-options.908912/#post-10194843
Conferr made a bench seat with storage under
They also had two other storage boxes that would go in the rear bed area
There is no other commercially available under seat storage for rear … and even then conferr has been out of business for many years
Agualu makes a drop storage
I think BTB...