The best approach I’ve seen was a friend who cut his factories in half with a hacksaw. Large factories are majors, while the smaller, halved factories are minors.
I’ve got mine painted three different colors. Bought a lot of 50 for a few dollars from a vendor at Nashcon in May 2016.
Before Nashcon, I used red Monopoly hotels for the major factories and the green hotels for minor factories. I still like to use the green ones every now and again because I think they’re nifty.