I wanted to mount my SR-602 pir in to a wall neatly so I made a 3D printed mount for the wall.
The sensor is intentionally set back in the tube to stop it triggering accidentally and you had to be close to the step.
The sensor just pushes in to the back to the 2 guide pins and seems to lock in to place.
Feel free to use and modify as you wish.
There are 2 versions, one were the PIR fixes from the back, other inside. With the inside version you can make a washers to step out the sensor.