This setup unfortunately adds a little inconvenience because you need to affix a magnet to the top of every new spice jar. To make it as painless as possible we have a little bag in our junk drawer with the glue and magnets ready to go.
Step 1: Materials
We ordered 100 3/8 in x 1/16 in Thick, Grade N42, Rare Earth Neodymium Disc Magnets from Magnet 4less for $9.00.
We went through a lot of different types of glues trying to find one that worked well bonding metal to plastic. I didn't want to use epoxy just to avoid mixing. We settled on plain old Krazy Glue and then Loctite Super Glue once we ran out of the former. They both worked equally as well.
We had 28 spice jars and with through a whole bottle of Krazy Glue and a half bottle of the Loctite. The Loctite had a much larger size (20g) than I could find for Krazy Glue (5g).
We ordered two custom-cut metal A366/1008 0.0359" (20 ga.) cold roll steel sheets from OnlineMetals.com for $28.51. Just a friendly reminder, make sure to measure twice before placing your metal order! Also, make sure to measure the area where you want to mount the metal, not the actual cabinet dimensions, and take a little off for a safety margin.
We went with cold roll over stainless for two reasons. The sheet would be mounted under the cabinet and not seen, so no need to get the "prettier" metal. Second, and most important, the cold roll steel has a much stronger attraction with magnets, compared to almost none with stainless steel. I think the cold roll can rust up, but I'm not sure how much of a problem that will become.
Screws and Washers
We had plenty of screws and washers left over from other projects so we just used the ones that looked best. Pick screws that are short enough that they don't go up through the bottom of the cabinet. If you don't have any short ones just cut a long one down and drill a pilot hole.