I did a little searching while I was eating lunch (working from home so I can access my PC easy) and what I found is you can make one with common hardware; cabinet hinges, piano hinges, childproof magnetic locks, RFID locks, etc. It just depends on how much you want to spend and how elaborate you want it to be.
Since I haven't found any plans, the challenging part will be "engineering" it. I wouldn't have a problem doing that.
The one hesitation I have is that if I go with the magnetic locking system, I have to have a magnet nearby to be able to unlock it. I can hide it in a decoration of some type, but it is sorta like leaving the key to the safe on the table. I'll need to think about it vs just buying a small safe.
If you watch the video on that web page you will see that not only are there two magnetic locks, she has to push up on the bottom of the hidden compartment (unlatch it) to get it to drop.
If it is hidden as discretely as shown in the video, do we really need the magnetic locks? (Sure they would be extra safety if there are kids around...but mine are all older and know better).