In reply to Horse:
I think it's more the rope tarp feature of rope bags that people are looking for (?) My mate's Outdoor Designs one seems to do the job on tarp and bag fronts, and is cheaper than the DMM.
I just bought 2m x 1.5m of proofed nylon and use whatever straps I have to hand. May try to cobble something a bit more bag-like when I get settled in the house. Also good for sheltering under in the pissing rain...