I would use 1/4" triangle balsa stock for the tail surfaces. Use four pieces, one on each corner where the fins meet the fuselage, top and bottom. See pic:
If one of the wingtips is heavier than the other, which usually the muffler side will be heavier,
I drilled a small hole in the flat, vertical part of the tip that faces out and squeezed some silicone caulking into the hole. Use just a little at a time and keep rechecking the balance by raising the model by the nose and bottom of the tail. Keep adding silicone until you get it to balance level. It won't take much, because the caulk is pretty heavy. Once it dries, it will be inside and out of sight and won't move around. Put a scrap piece of covering over the hole.
Don't do the balance checks until the model is completely put together with everything on it like it will be right before flying. Do the front to back balance first using the CG in the manual.
Fix that balance first before you do the side to side.
In the first and second pics, you can see where I would put the triangle balsa. In the third pic, the red dot is where I drilled a hole to put the silicone caulk in to balance. Drill the hole in line with the wing spar, so that adding the caulk won't mess up the front to back balance of the plane:


Jesse