Use 3M Blenderm tape if your single goal is to seal the gap for aerodynamic benefits. It's available at your local drug store. Also Du-Bro offers it so it may be at you local hobby shop as well. The tape sticks extremely well plus has stretch/flexible like properties.

If the above plus cosmetic cover-up is your goal use iron on Monokote or Coverite cut to size folded in V shape to fit the hinge groove then apply iron heat to seal the edges. A trim iron works great for this task allowing you to iron down inside the hinge groove.