I made some mistakes on this and the part just doesn't fit right as well. My mistake was installing the angles right on the outline of the lower skin. Because it slopes up and in I couldn't get it to sit on top of the fuselage properly, therefore making the gap worse. So my advice would be to set the angles in about 1/8" to 3/16" from your line. I think I finally figured out the problem with the way this part is made. When it is made the angles and sides of the skin are flat and perpendicular. When it's installed, the bottom where it meets the fuselage is curved as the sides curve out they need to be longer to lay against the fuselage, so the cut of the part should not be perpendicular but should be more elliptical. Anyway I have oprdered a new part from Zenith and will update this further.
A warning on this...when trimming don't trim the bottom middle edge (that lays against the fuselage) of each side, that is the area that needs to be longer, trim the front or the rear to get the fit and take off very little at a time. Four hands definitely helps on this, the metal is thick and it's not easy to get the sides pushed in and hold it in position and drill a hole at the same time.
So after that, I made the joggle braces that attach the bottom of the vertical fin to the fuselage. The part from Zenith is incorrect (may be solved by now) in that they were two flat pieces and the spar of the vertical fin is wider than the fuselage, so I made my own joggles parts....came out good.