I have never built an SOF boat, however I have a Shade designed Great Auk built as a stripper ( http://www.guillemot-kayaks.com/guillemot/node/28/draw ). I found this boat difficult to paddle in a straight line. As built, the boat had a tendency to "spin out". By that I mean that I could be paddling along and suddenly the stern seemed to be swinging through 30 degrees to the right or left and it would take 5 or 10 good hard correcting strokes to bring it back. Often I would over-correct and would find myself applying corrective strokes to the opposite side to finally get the boat headed on the proper course.
Accordingly, I added a fixed skeg to it. The dimensions of the skeg are 19" (48 cm) long, x 3" (7.5 cm) x 3 mm thick. It is positioned 12" (30cm) from the stern stem. The skeg is made from 3mm thick plywood and glassed both sides, then fastened to the hull with thickened epoxy filets and 1" glass tape.
The difference it has made in handling is profound. The boat is now directionally very strong tracking (difficult to turn.) I am inclined to think that I made the skeg too large.
As I interpret your description, your boat always turns to the right. This may indicate a number of things. However the one that comes to mind is that your keel line has a curvature to it. Sight down the keel line of the boat and if the keel has more than about 6 mm of curvature from bow to stern then this might be the source of your problem. It could be corrected with a skeg like mine, rather than resorting to extra hassle of a full keel. It will slow the boat a touch but you can make a skeg that is deeper than mine, then turn it a bit to give a touch of permanent, left rudder.
It occurs to me that you could make a temporary skeg from 1/8" acrylic plastic sheet and duck tape it to the boat near stern and go paddling on a calm day. If this fixes the problem on the first try consider yourself fortunate, otherwise adjust the alignment and size of the skeg accordingly. I have never built a SOF and cannot advise you on how you could attach anything permanently to the keel.
Other things that can affect the directional stability of a boat are paddling technique, weight distribution, and wind speed and direction.
Finally you may want to re-post this in the Builders Forum, you will get a lot more attention from more knowledgeable folks than me.