I've been brainstorming on how to better utilize the space behind the seat in a double cab and I've come up with an idea or two...
First, remove the factory plastic storage bins.
Second, have a sheet of aluminum cut to fit the space behind the seats with provisions to bolt or rivet the sheet into place.
Third, have a MOLLE panel custom manufactured to fit the space and attach it to the aluminum sheet. The MOLLE webbing offers ENDLESS storage/organization options using various pouches. The concept is already utilized well with accessories made for Jeeps and FJ Cruisers like so:
Lastly, at some point I'll add a law enforcement type gun lock for securely storing a shotgun or rifle horizontally behind the seat.
I'm also considering removing the under-seat storage bin and having a fab shop weld together a steel replacement with a locking lid for even more secure storage.