I have been playing around with ideas for a retractable skeg for a while and bottom line is there isn't a simple and good way to put a retractable skeg on an existing frame that I have come up with. You can add one but it creates so potential problems that I don’t like.

I have a boat with a lot of rocker designed that will incorporate a retractable skeg, but this boat is designed specifically for this. It's not an add on item, it is a change in construction methods.

The simplest way to add a skeg is to cut a 1/2" thick strip of wood and using a couple of wood screws, just screw it into the keel. You want to bed the mating surface with a good caulking to seal the holes of course.

I did this on one boat that had very loose tracking. I put it on with no caulk and started out a little large. I made mine long and not very tall. I trimmed it couple of time so get it to the size I wanted then I put a finish on it, caulked it and never had a problem.

You didn’t ask, but if you are not having tracking or weathercocking issues I think you are barking up the wrong tree. A skeg can make a boat track much harder and making it harder to turn too. I have never paddled this boat but it is a proven design and just by it’s length I suspect it is hard enough to turn as is.