I had mine made by The Canvas Store on long island. Google should find it for you if you are interested.
They made one for a buddy of mine that fit like a glove. The design is two part, fits over the boom and ties to the base of the stanchions, so it keeps water and snow off the boat but allows air circlulation. I think mine cost 1500 or so.
The owner and I had a lot of go-round getting it made, and it arrived late, in early December, despite my ordering it in April, so I froze my fingers off putting it on, but it is GREAT, and well worth the investment, although the aft half does not fit well; it is a little large.
The material is TOP GUN, which is great.
If you choose to get a quote from him, let me know, and you may freely quote my experience to him. I would love to have mine re-sized a little, but it is too aggravating. This year I just want to put it on and be done with it.
By the way, you can turn on a space heater under the cover, and basically work on your boat in a heated tent in winter. Very nice