Hi, any 12v Domestic PIR will work, have a look here... https://www.securitywarehouse.co.uk/catalog/intruder-alarm-c-36/movement-detectors-c-36_116/standard-pirs-c-36_116_118/?sort=4a&filter_id=&product_listing_sorter_id=1
We took over a company c2001 who had hundreds of Veritas out there, I had a patch with c800 contracts and used to service 2-3 daily. I never had an issue with the screw covers or mains clips, bit of a knack to removing them without trashing them I agree but I prefered the look with them
For my O level I made a weightlifting type bench, spring loaded leg contraption, it was blooming ace and in those days I was allowed into the DT room alone during lunch and after school to weld etc.
@Gabs I'll have a look in my garage tomorrow, I'm sure I have some bits and pieces that you could make use of. When I've checked I'll pm you. You can have them foc of course.