Back in 1977 I made my own magnetic drain plug for my Honda Accord
Got a cylindrical magnet, measured the OD and used a numbered drill just slightly smaller than the magnet and then pressed the magnet in.
After that, where I use working , they had an industrial magnetizer, which I inserted the whole drain plug and zapped it a couple of times and I did this to two drain plugs
Whenever I had an oil change I gave them a clean drain plug and they gave me the dirty one. Each time there was small quantity of powdered iron/steel on the magnet.