Hey Oldpath05- not sure if waterproof is essential- the switch I had probably isn't, but where it's mounted it's mostly shielded from rain and snow. And, I my case, I try to avoid rain on my tractor. Snow, if we ever get any again, would be another thing altogether. But I have a couple of those switches, so worse case, I replace it. The indicator light I used looks just like the truck marker light that was linked- can't remember why I even had one, but I did.
As for the horn, I've been thinking I might stick a spare horn on mine with a momentary push switch on the dash. Be easy, and there are rare times I could use it. I've replaced OEM Japanese car horns with louder, more manly horns, and the old ones could be re-purposed. Might not even need a relay- just 16 gage wire.
And yes, the voltmeter is handy- tell you a lot about the battery and charging system at a glance. I've got another one that plugs into cigarette lighter outlets in vehicles which I can use if I suspect something. Though I guess they aren't called cigarette lighter outlets anymore.