I just finished testing a prototype poly tarp (i am guessign 2ml thickness) using LDPE - that's low density polyethylene platic - supported with transparent duct tape. I have been recording the progress of this project and will include a link.

http://www.youtube.com/watch?v=FgC
Uc_aUnb0&list=UUxMuB1uhS6vBN_pFoYpe0Tw&feature=plcp

Here is another prototype test using .35ml High Density Polyethelene (please forgive my spelling). I like the HDPE because of its weight to strength characteristics, but I don't have a source for a .7ml version (without ordering a bulk roll of 400 ft.)

http://www.youtube.com/watch?v=6BuQ2X-oW10&feature=related

Hope this helps

Cheers