I have a GunVault mini (GV1000) installed in my truck. A buddy built a 1/4" steel plate (following the template provided with the safe) onto which the safe is bolted, with "arms" extending out the sides... these "arms" fit under the seat rail and securely hold the safe in place, under the same Torx bolts that secure the seat.
A side benefit: here in FL, they have the "10/20/LIFE" law -- commit a crime with a gun, 10 year minimum mandatory; use the gun, 20 year minimum mandatory; injure or kill anyone during the commission of the crime, life without parole. ...so, as soon as someone breaks into my truck, they've committed a crime (burglary) with a gun (once they illegally enter my vehicle, they are in constructive possession of the firearm in the safe, even if they can't touch it... or, even if they don't know it's there!), so sentencing starts with a minimum 10-year sentence.