The question is what are you really trying to do here?
If it's hiding a gun or whatever in a floating mantle, the overall design this basspro guy has is pretty good. I'd build one myself or have a trim carpenter do it. I'd forgo the childproof magnetic lock that needs the key for a few recessed keyless magnets so all you have to do is push down the hinged part of the mantle to open it, instead of looking for the key, or positioning the key in just the right spot.
Personally, I'm not a fan of mantles that are anywhere but above a fireplace.
An alternative would be to cut a hole in your wall behind a hanging mirror or picture. Frame in that hole with a box, hinge your painting or mirror, put a magnetic push latch on it, possibly a magnet to hold your gun in the right position, or I guess you could use a cheap holster, and there you go, hidden in plain sight. Think in wall safe. Better yet would be to install an in wall safe with a painting, mirror, or even a fake electrical panel in front of it, location dependent.
An outside wall will usually be 2" deeper than an interior wall.
Holster or Gunvault screwed under your coffee table? In a kitchen cabinet, behind your microwave or inside the fridge?
I guess it all depends on how paranoid/prepaired you need to be.
