A door that is completely invisible from the outside and can only be opened by throwing an item at the right spot.
To open the door, you need to throw an item on a certain block. However, this is only an entrance door. To close it behind you or above you, you simply take the item from the funnel from below.
Instructions:Uncovering the ground reveals the following:
Since you can see everything on the screenshot, there is no further explanation at this point - just a screenshot in case something is unclear:
The block with the funnel underneath is the one you have to throw the item onto. To find it faster and not have to count it, you can mark it with a different color from the top.
It is only important that the cover be half-steps must be.
Alternatively - if you want even greater security - you can still secure the door with the Item sorting machine combine, because then you can only open the door with a certain item.