You could use the one you've got, but it's going to be a challenge. Making it fit the curve of the mast
wll require more than a stout hammer, because beating it into shape could damage the weld. Instead, you would need to clamp one side in a vise and heat the other side to about 900 degrees F, or visibly glowing red in soft light, and then bend that side to the right curve, probably with an adjustable wrench. You'll need an oxy-acetylene torch to do that - propane
won't give you enough heat.
The ring you've got was designed for a larger boat
. The plastic one - well, Spot spoke with wisdom.