I fitted a pair of trumpets to mine, one high tone, one low tone.
https://www.beal.org.uk/automotive/horns-reversing/trumpet-horn-12v-high-tone.html
https://www.beal.org.uk/automotive/horns-reversing/trumpet-horn-12v-low-tone.html
If I recall, the Mazda horn has a single wire and earths through the body.
And I believe these trumpets had two connectors, so I had to make up a short fly lead back to the mounting bolt.
I did mine without removing the undertray, I just dropped the front lip, although it’s scratchy on your arms, so for the sake of 10 minutes, it’s probably worth dropping the tray.
I didn’t bother with a relay.
Sounds much better than the toy ones I took off.