Look on FleaBay for ATV tank bags. They will drape over the rear of the seat, but then it's a PITA to raise the seat to get at the pet carrier or gas. I ended up cutting them in half and then screwing them down to the rear fenders just below the seat. I keep all the "semi permanent" stuff in there (windbreaker, cable lock, bungee net, etc.) which leaves the pet carrier available for cargo.