Just looking at my car today - and yes I agree, not wise to chop anything. I reckon it's just as easy to remove the cover and replace the original siren unit with the more up to date version of the siren. There are the 7 screws to release the cover, and of course the wiper arms need to come off. These can be tight and difficult to remove, put a simple and cheap puller can be used to release them if needed.

I have had a little buzzer attached to the flasher relay in the fuse box for a couple years to get the desired bleep bleep bleep when locking. But the right way is to fit the undated siren and I will do this when i get my hands on one