I read a bunch of different techniques on how to do this. Some suggested using metal trays cut from metal roof flashing to having a machine shop cut the required pieces. I opted to try something else, magnetic paint! This took me awhile to find at Lowes because it was in a box. Who sells paint in a box when everything else around it are in cans? This can cost about $22 for a quart. I opted for the paint as I already have movement trays for every one of my armies and that way I should just need to put a few layers of this stuff on and be done with it.
Each coat takes about 30 minutes to dry, and even after 5 coats it barely held a plastic model on. Any shaking at all and the models would fall off. Metal figs just fell right off. The magnets I use are 1/4"x1/16" which I got 500 for $25 off ebay.
So I went and tried something else. I bought 3/16" washers which are about an 1" in diameter (same as a 25mm base). The only problem is that I never liked the gaps. So I used some normal household filler in all the gaps and just primed over it, painted, and flocked.
Now they hold even metal figs nice and secure. Even shaking won't knock these suckers off.
