This is a presentation/display case I had made for my father's retirement gift. It has two pretty conventional hasp-type closures on the side. These have locks built into them, so they can be secured at the owner's discretion. If your character keeps the case locked, he could simply break the glass to access the pistol, though breaking glass and reaching through it is seldom as blood-free as it is in the movies.
The pistol itself is held in place by pressure from the thick foam that is hidden under the red felt. This presses the pistol against the glass, and friction keeps the pistol from moving/sinking/falling.