Well, this is one way I did it, stringing the tarp along the mizzen, which I tied up with a halyard. That was so-so, and a lot of work.
Version 2.0 is a lot better: I just tied the tarp back from the dodger to the mizzen, made a kind of gasket joint around the mast, and then pulled the tarp corners down to the opposite quarters of the boat while tying the sides out to the gunnels from the dodger back to the mizzen. And I made up some tent poles to make U's underneath, from fiberglass "replacement" poles I found on the web. So that required sockets, but I could use my oarlocks. It all worked, notably in a roaring Wisconsin Thunderstorm. Not pretty, I suppose. I kept a lot of the tarp over the dodger to help insure rain-proof-ness. The tarp of which I speak is from REI, sold as a camp tarp, 12x12, lightweight, waterproof.