I use a combination of the MIK Carrier Plate MIK Carrier Plate which attaches to the bike rack on my Level and a Bottanger MIK Trunk rear basket Bottanger MIK rear basket though they are pricey. Also Basel MIK Cento basket MIK Cento Rear Basket. Search for MIK Bike Basket and see if any other brands have a basket with the Basel MIK Adapter Plate MIK Adapter Plate. Some other baskets with handles may have mounting holes in the bottom that will fit the MIK Adapter Plates so check with your local bike stores.
Another option is the Racktime SnapIt 2.0 system. Racktime products. This may require replacing the rack with a SnapIt rear rack.
Other systems by exists but these are the two I know about.