I remember the same thing happening to me on my first H16. The first time, the cast aluminum housing broke just like yours.. I re-drilled 4 holes and fixed it with aluminum rivets but it occurred again. This time the rivets sheared off.

What I learned with raising my H16 mast.... I now have a second person push down on the bottom of the mast to push it into the mast base housing. Or if I'm by myself, Once I get the mast up on my shoulder, I pull the lower part of the mast up and back with one hand as you lift the mast with the other. The 16 mast step is designed where the lever part can get pinched and the torque of raising the mast will rip the mast plate out of the mast... Using the H14 hole for the step pin helps a little but it is just a Bad design.

For this reason, aluminum rivets don't hold well.