Thanks for the info about silnylon being hard to work with. I don't have the skills to sew cotton, not to mention something slick and difficult to use...I'll cross that off the list for now.
How about the idea of using tyvek to create a bathtub? I've read it can be glued or taped. Having never worked with tyvek I don't know if it will hold the bathtub shape like silnylon (option 1 above). But this is the option I'm leaning toward because of my poor sewing skills and if it doesn't work I can always use it as a simple ground cloth (option 3 above).
The Cloudburst (made by Tarptent)I'm buying is $80, add another $20 for supplies to create a bathtub floor and I've got a 2 person, bug proof, rain "proof", 2.5 lbs (approx.) tent, which shaves 4 lbs off my pack. That's motivating. :)