I have a magnet stuck to the outside of the cover of everything that holds fluid that isn't aluminum. I'm hoping any debris inside gets attracted to the magnet through the cover. I suppose I'll find out the next time I pull one.
I keep some flexible plastic magnet material in the garage ( a 1/2" by two foot roll, cost a few bucks). And epoxy a piece someplace, out of the way, inside of the case. If it does ever come loose or migrate someplace, the chances of serious damage are minimized.