Keel If I were you I would forget all about adding a keel. It may make the boat track straigher but will never make it more stable. Learn to paddle better. That is your key. Not having a keel makes moving the boat sideways in a river much easier.
The creative possibilities are endless You don’t say how deep the T extrusion is. If it’s six inches deep, you will overcome GBG’s objections about insufficient depth to affect stability. However, with a 6" deep keel, you will no longer be able to sail in Assateague with Mike McCrea because the whole dang place is only 5" deep.
Contrary to Marc Ornstein’s fears, the T extrusion may strengthen the bottom of your canoe. The only way to test this will be to bash down a 60’ per mile descending shallow creek.
Of course you should attach it first temporarily with tape or velcro so you can remove it. This will also allow you to position the extrusion on top of your off-side gunwale vertically to present greater freeboard to an imminent tip. Alternatively, you could tape it on the side of your off-side gunwale to act as an anti-tipping shield.
Finally, you could just lay the enter 14’ section horizontally across the gunwales in front of you, with empty 5 gallon plastic jugs hanging off each end. This would act as a Polynesian double pontoon and allow any Magic to surf across the Molokai Channel.
Ahhh, the ideas that come to mind late at night when one is bored and is unable to sleep.
No, not the idea of adding a keel to a perfectly designed hull, but rather asking a question that is outlandish and then sitting back to see how thoroughly I am flamed.
You would think the boat alteration firm of Saults and McCrea could make a rudder that could do double duty as a rudder and a centerboard. That hole in the bottom of the Magic would take some clever engineering, no?