There are lots of Oil Filter magnets of one type or the other on the market, ranging from a disposable type ceramic magnet (you drop it inside the center channel of the spin-on filter where it sticks to the internals, then you throw it and the filter away) to the wrap-around and end-cap types. Their cost ranges from $1 (disposable type) to more than $60 for the end-cap type.
A filter magnet is a good thing... paying $60 for one isn't.
I use a rare earth magnetic disc measuring 1"x 0.125" . It attaches to the outside of the filter cannister. It has a Pull Force of 10.69lb (4849g), so once attached, it's not going anywhere and it is stronger than any of the other products out there so is sure to do as good, if not better, a job.
When it's time to change the oil, just pull off the magnet and attach it to the next filter. Best of all, they come in a 4-pak for $9.99.
I have been using them on all our cars for about 4 yrs and once cut the filter cannister open to see what the magnet actually captured - no chunks (thank God) but lots of ferrous goo, so I know it did it's job well.
See them here - Rare Earth Magnets
Cheers!