Everything must be secret here!
I started designing from this sentence.
Outside
The most important thing is how make a secret door without draw attract. So I used the available materials and designs.I designed the wall with the same paintings and wooden bars. Nobody can think maybe here is a door. As you can see from drawings I observed some tolerances to have a nice same levels.
Groove
I designed the groove with a light on top of that and some NOTHING.Because we have only 41 cm there and it's just enough for stand a person in there. But if you want, you can add secret closet there (It's not my idea and not be useful in somewhere is a WAY).The most important thing is a small step on ground.It;s for inside door.
Inside
As you can see I designed for you two alternatives: Mirror or a Board.It's based on your opinion.I talked about a step in groove, It's for making a level for inside door. Let's see what will happen without that:
1-the mirror must come down to ground (It's not common)
2- you will have (At least) 2 mm black strip on your wall ( Draw attract)
Handle Systems
As you can see from docs, we have two doors. for outside door I designed a simple decoration. It helps us to hide our handle. You can see on plans we have a small space ( 2 mm) that we can connect the wooden decoration and handle with a pin. So when you rotate the decoration you will rotate the handle. the other part is a pivot part that you get it up and use your key.
For inside door that's a simple system of openings you can see it in some dressing rooms.
It's simple, Cheap and Useful.
I hope you enjoy your secret door