OK. First, is the little cable in place connecting the big anode to the stern bracket? Are the facings between the anode and the stern bracket clean and have good metal to metal contact?
If all of that, I'm guessing that your steering cylinder my be sufficiently isolated from the engine brackets to not get the sacrificial benefit of the anodes.
I imagine that you can attach an external anode to the cylinder itself using very shallow bolts. You can purchase all kinds of anodes on line, in all kinds of shapes and sizes.
I was once told to never mix anode materials. However, on the Honda engines, the external anodes are zinc, and the internal ones in the manifolds are some sort of aluminum composite. Magnesium anodes should only be used in fresh water.
BTW - is your boat fiberglass?