Just a possibility but you might check with Truck Vault at
www.truckvault.com
I have one of their gun cabinet/storage systems in my F150 SCrew and when I bought it I could have sworn that they were making an interior storage system for under the back seat of a vehicle yet I see no obvious mention of it on their website now.
Might be worth a phone call to them. They'll build anything for you, just get out your checkbook...their stuff ain't cheap but it high quality.