I'm thinking of doing the same thing. I have too many guns for one real safe. I have a few RSCs and cabinets. An RSC you could open in 5 minutes with a Sawzall or big prybar. The cabinets would open with a screwdriver.
I'm thinking of putting them in a walk in closet. I'd line the closet with plywood and maybe an extra layer of drywall. Then I'd go with a solid core door and deadbolt. It would be just as secure as a RSC, in my opinion. Wife wants a safe room anyway.