Could see about getting one geared more towards police vehicles?
https://www.tufloc.com/professional-...cle-gun-racks/
https://santacruzgunlocks.com/
They have options for muzzle down, vertical and horizontal across the ceiling mounts. I would suggest if you go that way get the handcuff lock style. in my experience they are easier to use.
Of course you could also look for a safe under the seat like this.
http://bedgunsafe.com/truck-bunker/
Lots of options for safes under the back seats.