It's looking like the rain will come any time now, so I went ahead and took some photos of what I think might work to keep the heavy rain at bay, sort of. The first shot is of the TrailStar pitched low and taut, using a canted pole. By the way, how much cant can you get away with before something fails? Is mine too much, or can I do even more? An inverted V with 2 poles lashed together is probably the best arrangement, as far as creating a large sleeping space in the center of the tarp. But that implies that I would need to find the right stick to support the front opening. By the way, my binder clip idea failed miserably--the first big wind gust popped the clip right off the slippery silnylon.
Warning--this technique is something only a mother could love.
The next photo shows the front dropped to the ground, with the linelock pinned down to minimize flapping in the wind. There's plenty of sagging of the front 3 sides, and wind would play havoc with it, but at least the rain should be kept at bay.
The linelock is held down with a 13" version of the Stix I sell. I usually carry 2 of these and 4 regular length Stix. Here I am using the four regular 10" Stix to pin down the centers of the side panels. It's easy to release the linelock from the Stix to get out, and to secure it when I'm back inside after my quick little jaunt outside.
Keep in mind that here in the Rockies, where I do my camping, rain doesn't usually last very long, and all that is needed is a short-term fix to keep the bag protected during a big downpour.
I told you that this is a fairly crappy solution, but it should work OK. Stuart, I await the photo of your cuben door.