For anyone who's in the same boat as me. I spoke to Champion and they gave me the part number I need. It is K3784 and the Aircraft Spruce reference for that part is 08-01381.
That is for Slick 4370 and 4371 mags. I assume it is the same for all 4300 series mags but I would confirm with Champion first just in-case there is something funny with other mags.