Hi,
I’m thinking of creating a DIY alarm system using Aeotec PIR sensors and hub.
The only problem is that I can’t find anyway z-wave outdoor alarm sounders which you fix to the outside of the house.
I’m based in the U.K. if anyone has any recommendations.
not exactly the answer, just a hint.
If you can’t find such device, a Z-wave switch will be able to trigger any sounder… 
Aeotec Siren 6 may not be outdoor rated but could probably be mounted in a weatherproof enclosure.
The Doorbell 6 is the same hardware sold with RF doorbell buttons.