I have this one in the sides car and really like it..

Genuine Subaru SOA567T100 Cargo Organizer https://www.amazon.com/dp/B0081KAW18/ref=cm_sw_r_cp_apa_FzwVzbZYCDT40

It is collapsible however pretty sturdy. I put Velcro on the bottom to keep it from sliding around.

Another option is a largeish plastic toolbox. I have one that's 30" long by 12" wide and 14 inches high that I use for my car. Damned if I remember where I got it its almost 20 years old by now.