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.

I never thought if using velcro. It dosen't seem to me that it would have enough stregnth. I use a heavy duty rubber snubber to hold my large medical box up against the back seat. It doesn't move, but there is always something to rattle in there no matter how perfectly I organize it. Being how its right over the rear axle every bump makes a little rattle noise from the back.

I picked up what I use from the Home Depot. It is made of a heavy plastic material and reminds me somewhat of artificial turff, but shorter.
I tried a regular velcro that was self adhesive, but in the warm weather here, the glue ran a bit and it do not work that well. I am very satisified with this newer type.

If you have a large medical kit, you might need four pieces, one for each corner, but you would be surprised at how well it works.
But, I suppose it also depends on how well it mates with your rug or whatever it sits on.

For a long time before I started to store stuff under rear seat of my jeep I used heavy duty waxed fruit card box reinforced with duct tape and home made dividers. It was great for everything I needed and in case I needed to start the fire in emergency situation it was a life saver. Also lined with garbage bag it can be used as water storage or wash basin. Some of my friends swear by milk crates and USPS mail boxes (yes to have one is illegal).

Matt