I'm sorry I did not see this post sooner. I also have a Cal 21'. The keel plug design you have there is just what you need. Once installed you'll find that the footing you have will still fit over the keel well and, once the keel is lowered and the keeper in place, you'll be nice and dry.
When you make it up be sure to drill a hole to fit the bolt at the back of the keel well. It should stay in without that bolt being used but I did lose my first plug and wouldn't if I'd bolted it. I replaced my bolt with a better fitting one and use a wing nut to secure the plug.
Feel free to PM if you'd like.
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Wm. F. Buckley, Jr.