I take it this truck has the "split" seat, with both sides adjustable and a fold up or down center arm rest??
My Chevy work truck is so equipped and I hide my laptop bag behind the seat and set my daily newspaper over it.
Lots of room to mount one of these GaunVault
Best Car Gun Safes , with controls pointing up, in the same area. a simple reach behind would open the safe and provide a place to put it or use it if needed!.
A coat in cold weather and a newspaper in hot would be simple camoflage! as an added bonus you might be able to mount a steel plate to the outside of the cab, sandwiching the cab between it and the safe making removal by vandals even harder.
Just a thought to feed ideas for you. There are a lot of useful videos on this topic on Youtube. I'll leave one here for general information, I hope this helps someone.