I have used this motion detector. I think it is also available in white.
There are two little potentiometers inside to adjust sensitivity and the time it stays on (IIRC).
I open them up, figure out where the pots are, and drill access holes in the back of the case.
In my safe, they are zip-tied to square zip-tie anchors. You could also use double-sided tape to mount them.
https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1