The best ones to use are salvaged from hard drives. There are two of them in the hard drive, but only one can be used for the purpose of holding down material (at least in all the hdd’s I’ve disassembled.) You can buy them from ebay and presumably other sources. The reason for these is they have a shield on one side so the magnetic field won’t interfere with the fan head. That said, I mainly use straight neodymium magnets. The nickel, or whatever, plating cracks if they snap together too many times, but they work. I buy mine from K&J magnetics.
A while back using strong magnets could stop the glowforge head. Not because of the force of the magnets, but because they stopped the fan and the glowforge registered this as an error. Glowforge updated their software and I haven’t seen this as an issue since the update.
If you have a bulge in the middle of your material a magnet is pretty much your only solution. For holding down the edges are you aware of the hold down pins? You can print them yourself from scrap.