For sale is a Bearpawwd custom made cuben fiber bathtub floor.
The dimentions are 30" by 90" so it is big enough for sleeping pad plus some gear. It is made from .74oz cuben which is very strong and light.
I can't remember the exact weight but it was between 120-150 grams, very light.
You can see pictures here of the bathtub floors in SilNylon:
http://bearpawwd.com/shelters_floors/bathtub_floor.html
I used it under a pyramid tarp and it is really easy to set up- just stake the 4 corners
Looking for $150 including express postage anywhere in Australia