Here is a super simple addition to your trunk that will help keep groceries from rolling around.
Summary: I added some S hooks that can suspend a shopping bag.
Buy some S hooks. Mine came from the dollar store. 1" hook (so ~2" tall) with what seems to be 1/8" steel, 15 in a bag.
Find a drill motor and bit. The bit should be just larger than the S hook material, and sharp enough to get through sheet steel.
The towel is to collect chips.
Drill a series of holes for the S hook to go through. I used the stamped slots as the "entry" of the S hook, and the drilled hole as the "exit".
This step is a work in process. The hooks like to come off when the bags are lifted, so I taped around the mouth of the upper S. The tape is not load bearing, so any tape will do (masking is for easy removal)
And tada! Don't over load the bags.