A ferrous baseplate for a direct drop in fit under your single coils made from copper plated steel.
These plates can be attached onto the bottom of a Strat single (typically bridge) to fatten it up a bit - making it more similar to a Telecaster bridge pickup. These ferrous baseplates add low end response, meaning they can be brilliant on the bridge position pickup to remove icepick highs.
These can be fixed onto the bottom of a Strat coil by soldering a lead onto them and attaching it to the ground wire of your Strat pickup. They are magnetic so they can also be pushed on and held with the magnetism, but you may shake it off if you make sudden movements.