perhaps a look through here will give you some ideas:http://foc-u.co.uk/index.php/topic,5311.50.html
Most bikes Ive had the disc holder was mounted to the rear mudguard-ideal for getting soaked. Present Fazer has disc holder attached to front brake hose guide bracket. Have never come across a waterproof holder yet-every tax disc becomes wet then dries out gradually losing legibility until its time to buy a new one-don't get me started on the cost of bike tax!!!