Basically its just a shower curtain stitched to the bag of a rug - yes thats it! Why pay a fortune when you can buy a rug and shower curtain and make your own
So i just bought a fleece rug from Range for £4 and a shower curtain from Sainsburys for £4. I cut the shower curtain so it was about 5cm larger than the rug and pinned them together with the right sides together. I then machine sewed three sides entirely and then on the forth side i left a 15cm opening. I trimmed any excess off and turned everything out of the gap id left. I then hand sewed the gap shut. You could top stitch all the way round if you dont want to handstitch the opening.
I now have a lovely turquoise backed rug thats perfect for sitting on damp grass (something we have a lot of around here!)
It isnt totally waterproof as the stitch holes do let a bit of moisture through but its usually not a problem. If you want to make it totally waterproof you can paint on some of the sealant you use to seal tents