Consider a Truck Vault. The company builds them to fit all of the common SUVs used b law enforcement, and its probably the safest, most secure, least visible way to conceal long guns and other equipment. They're not cheap, probably running $600-800, but they're also relatively easy to install compared to racks.
I ran into that problem trying to find racks for AR-15s about seven years ago. Most of the traditional racks are designed for Crown Vics, Caprices, Impalas, Tahoes, but none for less visible unmarked vehicles such as Grand Prixs, Taurus (before the current interceptor), G6s, etc. It ended up getting expensive after finding a shop to fabricate the mounts so we could put them in trunks. Furthermore, I learned on a Nissan Altima that there really wasn't enough thick metal in the trunk floor or deck to screw into. The only place where the metal was thick enough was in the floor pan, and the fuel tank was directly below causing them not to want to drill there either. We don't run cages, and I've had reasonable luck putting a rack horizontally on the floor behind the front seats of SUVs and four door pickups. With intermediate sized vehicles, however, it kills your back seat leg room.
That's why I would say go for the Truck Vaults.