That sounds like a neat challenge. I’d start with a strong, straight‑grained hardwood—maybe maple or cherry—because the grain will hold the joints tight and hide any seams. For the frame I’d use mortise‑and‑tenon or dovetail joints; they lock together mechanically so no one can pry them apart. I’d fill the mortises with a high‑strength epoxy, then clamp them until fully cured. That makes the joints rock‑solid. Next, the lock mechanism. I’d hide a simple deadbolt inside the back panel, but reinforce that panel with a steel plate that’s set flush under a thin wood overlay. That way the plate is protected by the grain and the lock is hard to reach. I’d also add a secondary latch that only opens from the inside, so an intruder can’t simply bolt the door shut. Finish the surface with a few coats of varnish to seal the grain and make scratches less obvious. If you want a subtle security cue, you can stain a thin strip on the edge that only shows up under UV light—just a little secret for you. Overall, keep the joints tight, the material straight‑grained, and the reinforcement hidden. That gives you a beautiful, hard‑to‑breach piece that still feels like a true work of wood.