Just mount it on the side of one of the main frame rails - towards the front of the bike, below the front section of the tank. Probably on the RHS of the bike, but makes little difference waht side its on.
So how far off vertical can it be and still work? Instructions say it must point down at the track with no metal etc underneath it. Does the orientation matter at all?
At Assen all the Dutch guys have them mounted horizontally just like Dallass Ex British Superstock bike does and that crosses the line a bit quicker than our DD bikes do, so a small amount off the vertical shouldn't cause a problem