Cut two wedges from a cardboard box, with the appropriate angle on them. Sit your fuselage on the bench with the wings level, place the card wedges under the tail halves, and they'll both be at the correct angle from horizontal. More importantly, they'll both be at the same angle. If the dihedral is a couple of degrees out, it won't make that much difference - but if one side has more dihedral than the other, it will.