Rory,
The yard supplied me one, which actually came from Rain and Sun (have used more than one of their covers on dinghies for several years, and am happy with quality, as well as the fit).
BRe mooring cover was £345 + £10 P&P plus VAT (last winter). This is a mast-up cover that can be used on the water (no "belly straps"); can also be used ashore.
Mine on a BRe, but VERY similar to that in Rob's photo.
On Bre, there is a hole in the middle of the cover, so that the main sheet can remain attached to the boom (I don't leave sheets rigged if I am leaving the boat for a couple of weeks, as I am often forced to do). An elastic strap supports the cover, from the boom.
You will need to fit "buttons" or similar down the sides of the coamings, to secure the elastics to.
Has worked well for me, on a semi-drying river mooring, and has withstood several full gales. Expensive(ish) but would buy another.
Your idea, to fit over boom and main, would result in enormous windage; imo certainly not suitable for a mooring. Pontoon maybe ok?
Certainly keeps the gulls/ducks etc out (thank god we don't have that many/adventurous seals!)
I have a "Gull Sweep" (also recommended) rigged up on a pipe, that I set up in the "mizzen hole" and that keeps the blighters off the nice soft after end of the cover. Gulls generally land into-wind, so with the boat on a swing-mooring, the exposed aft end makes an ideal roost, if not protected.