I'm a big fan of rare earth magnets. I bought a bunch of them in different sizes. They are really useful.
I stick the nickel size magnets in various places in my farm Jeep to hold tools. One on the dash holds a pair of pruning clippers, and another a pair of fence pliers. I have some smaller ones to keep the shower curtain in place around the cast iron tub. Need to hide a car or key? A little J&B Weld glues a magnet to the key and it will never fall off, and you can hide it in places no other container or piece of wire will work.
These things are way more powerful than they look. The fence pliers are pretty heavy, and stuck vertically on the dash, they don't budge.
I happened to see this magnet for sale on eBay. "Antipodal Rare Earth 700 Pound Huge Monster Neodymium Super Magnet" This is one humongous magnet! They get stronger as they get larger, and if a couple of small one can draw blood if they get too close together and pinch you, I can only imagine the power of this baby!
The first thing that crossed my mind was.......What if it got too close to the side of the semi trailer & got stuck? How would you ever get it off?