I use Business Card Magnets that I buy at Staples, but I assume you can find them at most office supply stores.
They are about 4"x2" with a little extra on the edges giving you enough to do pretty much any sized model relatively easily.
They are also plenty strong to hold plastic models or plastic/metal mixed models.
Pure metal models especially really heavy ones won't be held well, so Rare earth Magnets will be best for them.
Fortunately most infantry and cavalry have at least 50% plastic now, so these Business Card Magnets work great!
I typically put a strip of glue around the base of the model and set it on the magnet, then cut them to size once th emodel is attached firmly.
I then will add sheet metal (Tin in most cases) to the movement tray and extra magnets to the bottom of the tray so it'll hold to a Steel tool box which is my carying case.
Magnetic models, trays and metal cases make for very fast setup of armies, and many times they take up th esame amount of space or even less than the typical foam carrying case.!