So I found a great option for adding a skeg to the TK-1. It's from a company that made another kayak that I was originally looking at. Sea Eagle.
They make a directional stabilizer (skeg) for their Explorer line of kayaks that looks like it would work perfectly with the TK-1. What's better is that before 2004 their boats didn't have a mounting spot for this stabilizer, so they sell it with a glue/patch kit that mounts it to really just about any boat.
It's adjustable, removable, and seems to look pretty good. So I'd be able to put it on the TK-1 without having any kind of home made solutions that look all ghetto.
This is the stabilizer here, attached to a Sea Eagle boat.
This stabilizer that I found has turned me on to another option for my next kayak, as well. The Sea Eagle Explorer 340X seems to be virtually identical to my TK-1...only longer so it would track better and go faster. It appears to be made very well and I like the looks of it. It's more expensive than the Cordova that I had been looking at, but it seems to be more of a logical choice for a next boat. That, and it's got a three year warranty which is nice!!! Here's the 340X as sold on Sea Eagle's website, with all of the accessories that come with it.
You can tell by just glancing at it that it's design is similar to the TK-1. It can be paddled as a solo or a tandem. It also comes with the paddles and the foot pump. I already have all of those, but I guess some extra ones wouldn't be too much of an issue. The second pump would allow Cathy to inflate her boat at the same time as me, and the extra paddles would come in handy in case we damaged the ones we have now.
I may have found a winner with the Sea Eagle 340X...