Here are some of the mounts I use to house HC-SR501 PIR Sensor
Single Sensor Ceiling mount
This is used for attaching sesor on ceiling
Double Sensor Ceiling mount
Used to mount 2 sensors at 45 degrees, may be on a walk way.
Single Sensor Corner mount

Use to mount on 90mm Ceiling cornice
and some others I collected from Thingiverse(Couldn’t find the links again)
Get it here