Here is my simple method. Bolt it on, or screw actually. If your building it and it sounds like you are. Inside at the stern just add block in the bottom of the bolt that is large enough for screws. to be driven into.

On the bottom make a skeg wide enough to run screws through and round the edges. Bed the joint at the skeg and boat with something removable.

Paddle it see what you think. I start with a skeg that is to large and then cut it down a few times till find the point I like.

You can make a temporary one and once you get it like you want use it as pattern for the final version. Then plug fill the screw holes, bed it in caulk, add any finish and away you go. IN your case you will probably want to glass it on.

I did this on a SOF and it took me 3 times to get the skeg where I wanted. I made mine about 12" long and rather low profile so it didn't catch on stuff. I didn't want a fin, to much leverage against it. Long and this isn't as likely to get broken when handling the boat on shore.