Keeping a supply of anything in the trunk of your car can get cluttered and messy. If you have a carpted trunk and plastic containers, or anything that will slide on the carpeting, keeping everything in its place can be a problem.
I purchased some heavy duty self adhesive velcro fastners and put one half on the bottom of the items I wanted to hold down.
The carpeting acts as the mate, and the weight of the objects keep everything in place. You just have to use the correct piece.
It isn't fool proof, but it helps keep things organized and easily removeable from the trunk. It holds a toolbox in place, first aid kit, and even prevents a fire extinguisher from rolling around.