When I saw the Wolfman Teton Saddle bags, they immediately reminded me of the Army's JSLIST bag. They're both made from water resistant nylon and have a roll top. The JSLIST bag has two shoulder straps that can be sewn together (or adpated for buckles) and draped over the seat. The only trick would be figuring out how to attach it to racks (bungies?). I'm not in a position to test them on my WRR for a few months, but thought I'd mention it for others that like economical homebrew solutions.
FWIW, the JSLIST bag seems to be slightly larger:
The Wolfman bags are advertised as 13" Width, 13" Height, 7" Depth
A JSLIST bag is advertised as 17" Width, 16" Height, 8" Depth
Front / open
Front / closed