Mag plugs are nice, but a mag on the outside of your filter is a whole lot easier, doesn't require 'cleaning' and can be less expensive to purchase.
They do make 'filter mags' that wrap around a portion of the filters exterior, but those can get a bit pricy compared to 'loose' rare earth 'general purpose' mags.
Last bike was a Stratoliner with filter mounted sideways directly behind front tire. I had a 1 X 2 X 1/2 neo (rare earth) bar mag placed on the underneath side. Dang thing was rated at about 80 lbs of pull,,, so strong it always wanted to 'grab' my wrenchs away from me! LOL!
It was always picking up metal filings/debris from the road, and I 'collected' what appeared to be a heavy duty drum brake spring from an 18 wheeler once!
The Strat had an aluminum frame, and I had hoped the mag would help with activating traffic lights,,,, Eh, didn't seem to matter.
I have heard credit cards and certain electronic items aren't fond of magnets. Fun to play with, but some of the rare earth mags can cause injury if ya arent careful! I had 3 of the bar mags and the wife decided to play with 'em,,, 2 collided so hard they sent small bits of shrapnel flyin.