After many weeks of searching, I finally found the perfect boom tent for PocketShip. I used it on a four-day cruise last spring and it fits as if custom-cut for the boat.
It's called the "Pro Fly Rain Tarp" by Eagles Nest Outfitters. It's meant as a rain fly for hammocks, but the fit is just right. It stretches between the mast and the boom gallows and is slightly wider than the boat, providing excellent cover for both cockpit and companionway. I rigged mine above the boom for headroom (you can stand in the companionway), but in truly lousy weather you might set it beneath the boom to reduce the windage and the ability of rain to reach the companionway.
Here's a link to the tarp, available in five colors: http://www.amazon.com/gp/product/B00K30IPCM
Critically, it's made of nylon. Stiff polytarp is extremely noisy in any kind of wind and doesn't pack down nearly as well. This one is quiet and packs into a tiny bag.
You'll need poles at the front and back to extend the tent out to the sides. I built a pair from solid timber with a sturdy hinge in the middle for easy stowage. They assume a shallow "V" shape when unfolded. They look like this:
However, I wasn't all that happy with that solution, and found that the extendable aluminum poles sold in marine catalogs specifically for boom tents were easier to set up and offered more adjustability. The aluminum boom tent poles look like this, visible in the upper right:
Bonus anchorage scheme: I made up this simple companionway cover so that I could leave the companionway open on cool, windy nights. It's just a scrap of an old Dacron sail, cut to a 53" x 40" rectangle. A 44-inch stick in a sleeve supports the aft end, with guys running down to the spinnaker blocks. The forward corners are lashed to the chain plates. Takes maybe three minutes to set up.
Eventually I'll get all of this and many other "Cruising in PocketShip" suggestions compiled into a blog post.