I used a soft one like this before. It works with a car with no rack. Has 4 hooks that catch under the rain gutter/door opening. Worked great. There are different sizes.

WESTLEY Rooftop Cargo Carrier 15 Cubic Feet, 10 Straps Car Roof Bag Set with 6 Hooks & 1 Anti-Slip Mat & 1 Storage Bag, Waterproof Vehicle Cargo Carrier for Universal Cars with Rack or Without Rack https://smile.amazon.com/dp/B096WSBH11/ref=cm_sw_r_apan_glt_fabc_WMJC9PH5502GKQFHHV85?_encoding=UTF8&psc=1