A simple key latch mechanism. The key is a triangle shape. Only that shape will fit into the lock and unlock the mechanism.
This is not a perfect lock, as locks only keep out honest people.

Ver 2
Updated the Key, Triangle was too big when printed.

Print instructions

Category: Hobby Print Settings

Printer Brand: Prusa

Printer: i3 MK3

Rafts: Doesn't Matter

Supports: Yes

Resolution: .15 to .2mm

Infill: 15-20%

Notes:

I used Fill pattern of Grid. Rectilinear might give it a bit more strength in the key.
Might want to consider using 3 Perimeters for the key to make it stronger; found using 2 caused the tube part to snap when people played with the key.

Only need supports on the Latch part.

When printing the key, print it on the triangle end, or print it on it's side with supports. You might want to use a brim for better support while printing. MK3 did fine without a brim.