A standard rear lugage rack with a pair of panniers will carry a large and heavy load of groceries. Take care with heel clearance: my panniers have a cutout profile on the front edge for extra clearance. You need stiff, tough material, goos stiffening board on the back and base and for safety, a locking, quick-release mounting system. Hook and elastic are used on cheaper bags but are totaly obscelete and can bounce off.