It appears MagHold has stopped making them which is a bummer.. There are quite a few more competing products these days you can search for, but I really liked the simplicity and durability of spring steel + PEI.
I have actually started making my own magnetic build platforms for myself and should do a tutorial on that someday.. But the gist of it is:
Buy this spring steel plate (or similar from other vendor):https://www.mcmaster.com/9072k12
Buy this PEI sheet: https://amzn.to/2Cf50M3
Buy this 3M adhesive sheet: https://amzn.to/2CeI67t
Buy these super strong 20×5 magnets at least 20 pcs: https://www.aliexpress.com/item/20pcs-20-5-Strong-Disc-Magnets-Dia-20mm-x-5mm-Rare-Earth-Neodymium-Magnet-20×5-20mm/32844950305.html
Cut the spring steel to size with an angle grinder cutoff wheel, do it slowly to not overheat and loose temper, it is too hard to cut any other way.
carefully apply the 3M adhesive and the PEI – search for youtube videos for hints.
Attach the magnets to the bottom of the bed with Kapton tape. You can also use binder clips on the edges if the bed is too thick for the magnets to do much.