I don't keep my boat on the water, so cover is simply a 20x12 tarp over the mast. I've used the white ones with good success, but looks like the store I get them from has switched suppliers, the new ones don't have the same "feel", will see how they work out. I've gotten multiple seasons out of them.
When you are on a mooring, you have a different configuration, because the geometry becomes trickier. The boat still has some of the snaps attached where a previous owner had a cockpit cover attached. (Around the front of the cuddy). How this was held up (boom?) can't be reconstructed. The same owner left snaps on the seats, so must have had bench cushions.
If you want the convenience of a sail cover (so that the sail can be stored on the boom) I would go for a boom-tent that attaches below the boom, rather than being stretched over it. Some people have succeeded in adapting the outer shells of recreational tents for the purpose. Even attaching to the boom from below allows some tension to be set up to stretch the fabric. If you need to use it for camping, you might bring some of those bendable fiberglass poles to turn the triangle profile into a half-round.
Many solutions like that could be quite functional and because you'd be adapting something that's mass-produced, could be cheaper than any custom job.
By request, the discussion of tents for sleeping has been split and moved to:Sleeping on board, revisited..