I have been thinking about this for some time now, and still haven't come up with a DIY design that I like. Is Dupont's Tyvek acceptable as a membrane? I haven't yet tested the little piece I used grand larceny to obtain from Home Depot. What other easily available membranes are there? I'm still thinking about this, and have done several sketches only to dump them as impractical. Having only hand tools is sometimes a handicap for tiny little things like this.