I don't care for using magnets in molds. For the most part its unnecessary. Once you get a feel for the mold it gets to be pretty easy to slide a hook or a keeper into its slot. Even if you have to wear glasses like I have had to for the last decade.
Sometimes though the only way to get a piece of hardware to stay in place is with a magnet. This leads me to why I felt the need to post this. I learned recently that rare earth magnets have a failing. Above a certain temperature they loose their magnetism and that temperature is much lower than you might think. A little higher and they lose their magnetism permanently. These temps are below the melting temperature of lead. Now a mold rarely gets as hot as lead. Even if you set it on top of the lead pot to preheat, but it does get pretty hot. Hot enough that you have to wear gloves to handle it safely. Maybe closer than you might like to degmagnetizing temperature of the magnet.
I didn't research other magnets as I like the strong holding force of even tiny rare earth (neodymium) magnets for these problematic applications. I discovered that for a little more money there were higher heat rated rare earth magnets. I check a couple sources. MSC didn't have any in a size I wanted. McMaster had one size that was useful. K&J had them much cheaper than McMaster, but they were further away and only shipped UPS. I ordered from both McMaster and K&J Magnetics.
I actually had two molds that I couldn't come up with any other solution for. The first was done with the McMaster magnets and like other magnets I have ordered they were a little under their specified dimensions. They also did not seem to have a hard coating like most rare earth mags I have bought in the past. I just glued them in place with JB weld. The K&J magnets came in a few days later, and I miced them. I took measurements on 2 or 3 out of each pack. They were all within 0.0005" of target diameter, and about the same for target length. They also had the nice shiny nickel coating I am used to seeing. I machined tapered holes (.001 over at the top to .001 under at the bottom) to press the magnets into and they fit perfectly. I expect if you rapped the mold sharply on the bench you could knock them free, but neodymium magnets are pretty fragile. I didn't want to break them when pressing them in.
Another thing learned. I hope it helps somebody else.