I would look for an old style upright school locker. You could fit it in a closet if you wanted to and can fortify it and line the inside in a variety of ways. You could easily lag it to wall studs, the floor, a foundation etc.
Depending on what you’re storing, you could run a lag style eye bolt through the back into a stud and run a cable through it and lock your weapons up inside with that, in addition to the lock on the outside.
The possibilities of adding security features to it are endless, and in the end you don’t have something that advertises there are firearms inside a cheaper style safe.