My dad put one on his Pearson 34 back in Maine after their first run down the ICW. The first year they froze (left Maine in October). My understanding is they were comfortable with a fleece or sweatshirt the second trip even when the temperatures were in the low 30's outside the enclosure.
Don't know where to point you to but I'm sure there's a canvas shop somewhere around Anacortes that could do the job.