We made a (very large) two person tarp out of spinnaker (sail cloth). It has bug-netting which goes to the ground, and a floor which attaches with velcro. We tested it out in Henry Coe State Park, and you can find a picture of it here.
The tarp worked out really well on the Colorado Trail. The only night that we weren't 100% dry was the final night when it had been raining for 24 hours and condensation built up on the fabric. Falling raindrops would then knock a fine mist of condensation down on us - but it was quite minor and it was really wet out. I think that any single wall tent would have had problems in that environment.
Life has been amazingly busy since we got back from the CT. Consequently, we haven't written up the directions for making the tarp, although all the measurements and pictures taken during the construction are saved away. So once we get set up in Baltimore after our summer of wandering, we will get that written up and posted.