There are several types of fixed fins that could be attached to the bottom of your PDRacer, such as a ballasted keel, unballasted bilge board(s) and skegs.
(Note: see the bottom of this page for explanation on difference between a skeg, and a skid)
We are lucky in the fact that our boat has flat sides, so the simplest of solutions is to bolt the fins on the sides of the hull anywhere we want.
If you tried that on a curved sided hull, it would form either a scoop or wedge, and make large amounts of hydrodynamic resistance.
But for this essay, lets say you would like to mount a fin towards the middle of the hull where there aren't any convenient places to bolt onto.
External Chine Log
The first and most obvious method is to make a fin that has a top edge which matches the curve of the hull.
Then you can simply put a couple of chine logs on it, and glue / screw it on.
For more info on chine log construction [click here]
For info on using the spline method to mark the curve [click here]
To see how to match an inside curve with a tick stick [click here]
Internal Chine Block
The second method is using the same chine log principle, but instead you build up a bigger block on the inside of the hull.
1 - Make your chine block by glueing and screwing several thin strips of wood onto the inside of the hull.
The thin strips will bend easier, and after stacked up will provide enough of a purchase to glue against.
2 - Cut a slot in all the way thru the hull & the chine block.
3 - Glue your fin into the slot, also put a fillet of epoxy in the corner between the fin and hull.
There are several advantages to this type of joint.
First is that you don't have to cut a curve in your fit which matches the hull shape.
Next is that the part which protrudes into the hull could be a support for a false floor, which could make the hull self draining.
Finally when you break the board and need to replace it, just cut the old fin off and sand it flush, then cut a new slot and glue in the next fin.
There are skegs, skids, and stringers, which seem to be confused often. Here is an explanation to help clear up communication:
Skeg - This is a small fixed fin, usually at the stern which provides directional stability, meaning it helps keep the boat sailing in a straight line.
Many kayaks have them to help counteract the turning motion of the paddle strokes.
The Dovekie sailboat has a skeg at the bow to help it's directional stability, and when tacking that skeg is raised with a pendant line.
The Squeezebox, designed by Jeff Gilbert, has 2 skegs (one at the bow, one at the stern) instead of a keel or other type of leeboard.
Skid - This is a thin strip of board that runs lengthwise on the outside bottom of the hull.
It's purpose is to protect the hull, so when ever you run up on the beach or ramp, the skid takes all the abuse.
Stringer - This is a thin strip of board that runs on the inside of the hull, either on the bottom or sides, and can be running in any direction.
It's purpose is to stiffen up whatever it is attached to, just like a thick gunnel will strengthen the sides of a boat.
External Chine Log
Internal Chine Block