Australia

Sorry, I was trying to focus specifically on the means by which they are elected. Not trying to mislead, I think there's parallel because like the voice just says it has to be a body and not how it's made up and senate elections more or less just say "you have to have them, figure out how".

So there's a lot of leeway in the constitution in general, like to my knowledge there's nothing stopping senate electing laws being like a 2 hour window in the state capital except that if a government tried to do that presumably they'd be challenged on either going against the intention of the constitution, established practiced, or worse case there would be revolts. There's a few risks to making something too specific, what if things change and you need to like have another referendum to say hire a new person?

It certainly is vulnerable to say appointing the board of PwC as the voice, except of course that's the status quo now (inasmuch as advisory bodies on indigenous affairs go) although nobody could argue that the government was going against the spirit of the constitution doing so. So this can really only be thought of as massively strengthening the situation we have now.

If we look at how divisive even this change is, you can imagine the sort of polemic criticism a more prescriptive change might apply. If the voice fails to be satisfactory and vulnerable to being hijacked then we can always go to referendum again, we wouldn't be worse off than we are now. Legislation is a lot more flexible, which is a weakness and a strength but in general keeping systems flexible helps us fix things and keep them relevant as times and goals change. If the people want this, then the people have an interest in it working correctly after all.