Are you sure your tail is on straight? Mine was not, and it is not the only one like that.
1) Go to Home Depot and rent a surveying transit for the day.
2) Park the airplane on a smooth, level surface. Make sure the tires are evenly inflated, or do whatever else you need to in order to get each wingtip the same distance off the ground.
3) Set up the transit so its plumb bob is on the extended centerline of the fuselage. Here's one way to do it.
3a) Drop a plumb bob from the center of the nose to the ground. This may not be the tip of the spinner. In my case I used the centerline hole on the front of the firewall. Of course, that was easy since my firewall insulation was not yet installed. Try the center of the nose tire when it is pointed straight ahead. (You are trying to not include any offset due to a canted engine mount. Since the nosewheel is attached at the aft end of the engine mount, any offset will be negligible.)
3b) Drop a plumb bob from the reference hole at the aft end of the fuselage just forward of the rudder. If it's been painted over, find the center of the rudder cove, or the lower hinge pin, or the tail tiedown, or just estimate where the center of the tailcone is.
3c) Draw a line between those points and extend it a fuselage length or two in front of the airplane. You can use a piece of string, anchor it at the mark on the ground under the rudder and move the other end (out in front of the aircraft) until it overlays the spot under the nose. Yeah, I know, the tire is in the way. So go three inches to one side at all three points.
4) Look at the airplane thru the transit and center the reticle (the cross hairs) on the centerline of the airplane. Remember, it may not be the spinner tip. Examine the vertical fin. Is the centerline of the transit lined up with the center of the fin? If not, your fin is not vertical. Can you see more of the fin on one side than the other. This probably means that your fin is twisted. Mine was.
5A) You can fix the twist by marking the centerline on the fin, and using a contour gauge used for making cove moldings (available at Home Depot) measure the airfoil and make a "mold" of the most convex side. (Particle board on edge and bondo) This becomes an airfoil contour for the side of the fin. Fill the low side in with micro until the fin is symmetric. This is what I did and the pictures are below. The wierd looking picture is the view of the vertical fin from the front of the aircraft. It looks symmetric but white on one side. That's the micro you can see in the other picture. I think the micro was 1/4 inch thick at the top. It may not be a NACA 0012 airfoil anymore, but it will be symmetric, and plenty good enough.
5B) Since your airplane is flying, a previously recommended approach is better. Put a rudder trim tab on it and fly with the rudder deflected a tiny bit. So it costs you one knot of airspeed. Who cares? You can use a piece of aluminum like a Cessna, or you can attach a triangular wedge on one side of the rudder. I would recommed a six inch length of balsa wood 1 x 0.25 in taped vertically onto the trailing edge of the rudder. Taper the balsa first so it is 0.25 thick at the back end and zero thickness at the front. Go fly. Adjust the length of the balsa wood until you get what you want. Recreate in foam covered by 1 or 2 BID. Recheck the balance of the rudder.
Best of luck. All this work is really aimed at the person who is still building. If you are flying, it's usually simpler to use trim tabs and rigging adjustments if you can.
-Rob Wolf