This magnetic rack is specifically designed for the Vertuoline Pods. Each rack is 220mm long and you can fit 3 on the bed of an Ender 3. The neodymium disc magnets measure 8x3mm. Similar to these https://www.amazon.com/MB-THISTAR-Neodymium-Strong-Fridge-Magnet/dp/B07XC4G8Z3/ref=sr_1_4?dchild=1&keywords=neodymium+magnet+3x8&qid=1574280990&s=industrial&sr=1-4

It should be a tight fit and you may have to use a pair of pliers to snap them into the recesses. Using some waste plastic to protect the new print while fitting the magnet is recommended. You also want to put the batteries in before you remove the support material from inside the overhang so you won't break it with pressure from the pliers. If on your print the magnet is less tight, superglue should hold them. Alternating the poles of the magnets (as shown in the pic) will help assist multiple racks to sit snuggly together.

I hope you find this "Thing" as useful as I am.