Here's the bottom of the turret. Without modification, there isn't enough plastic around the edge to drill magnets into, so we need to build up the sides with something. I decided to use green stuff.
From the Warp: Magnetising the Space Marine STC, but I decided to try something different for the main turret which I think has been pretty successful. Only one more Predator and a Landraider to go and I can stop gluing magnets to my fingers and do some painting!