An good tarptent to work from or "copy" is the LunarSolo/Duo by SixMoonDesigns

You don't have to bother with the main curve of the Rainbow fly, and you can manage with straight edged panels (though a bit of catenary is better).

I have made several tents quite similar to this model, with sewn-in tub floors, and zippered doors.







All use one single pole or hiking pole for support, and are an irregular pentagonal shape.

You can keep the design simpler with a separate ground sheet, and a "curtain" mosquito door and sides.

Google Sketchup is a free and very useful application to start custom designing your own shelter.