How are you at building furniture?
Sometimes it pays to go through museums and see what other people have done to secure valuables.
One I recall looked like a simple 4-drawer "dresser" but had some interesting features. First was that the top drawer was set about 3" down from the overhanging top. This was because a concealed release allowed you to open the top (hinged at rear via a piano hinge) and access a large 3.5" tall compartment.
Another feature was that the drawers were slightly shallow for the depth of the dresser. This was because there was a 2" deep compartment behind the top 2 drawers. If you removed the bottom drawer, you did not see the floor. This was because the entire front "trim" panel concealed a drawer with a hidden release.
I've seen a grandfather clock with a concealed side panel that would hold a 22" barreled shotgun. Great for unexpected latenight guests!
In an antique store, the woman owner had a beautiful art-deco style desk. The top right drawer looked to be a locking drawer and when unlocked and pulled, revealed a drawer... but when a small button was pushed, it locked the drawer and the front "tipped out" revealing a custom made pistol compartment. She didn't understand why there was a divider with two slots in it, until I explained they would hold a pair of pistols nicely (1911's from the look). She was "shocked".
I've seen a round-topped cedar chest that was about 3ft wide and beautifuly made, with a false bottom. Looking at it, it has short "feet" and a hollow space underneath the chest. Tap on the bottom of the chest and it sounds firm, not hollow. Empty the chest and you can see the cedar bottom is held down by the 4 sides of the chest, no way to lift the bottom out of it. But grab the lifting handle on one side and pull down
and it releases a drawer just thin enough to accomodate his wife's 20-gauge Ithaca shotgun with an 18" barrel.