Montana Tag. The water is fine so far.
I use Bennett Law Office like Mike suggested and they are pro's at setting this type of thing up to be all legal. They got my existing business licenced to do business IN MONTANA so that it satisfies the letter of the law. I don't tow a vehicle so the aspect of the tow vehicle matching didn't come into play. I've been ID's by a half dozen police in the last 2.5 years since I did it and it has never even come up as a question as to why the tag differs from my California isssued drivers licence. Not so much as a mention.