I got these:
I figured I would need a 25mm diameter one but couldn't easily get one so went for the 27mm. It is a bit big but does still fit in the case.
The volume wasn't too bad with the back of the case off but it is very quiet once reassembled. TBH I'm not sure it's worth the effort, especially when you think most smart watches only have a buzzer anyhow.
Still, it's an easy fix for those that want to.