I put this together because I was tired of having my groceries spill all over the trunk every time I took a corner.
Step 1: Shoe Boxes
Basically, my design is just three boot-sized shoe boxes from my wife's closet jammed into the trunk. The three sizes allow for different sized loads to be carried without spilling. If you don't have a wife or boots, I hear that you can ask shoe stores for old boxes or use other boxes.
Step 2: Place Them in Your Trunk
Make sure they don't move around. If they don't fit together nicely, you may want to consider taping them together or putting some filler in between them.
Step 3: Insert Groceries
Now you're ready to go shopping! Revel in the glory of not having to pick up a dozen cans from your spilled grocery bags! Also, the milk will stay upright. It would suck to have it spill all over your trunk.