There is a wood batten strip behind the top edge of the MGA inner door panel. The wood strip is tapered to space the top of the panel inward at an angle to meet flush at the inside top edge of the door skin. If you buy a commercial door panel kit this wood strip should come attached to the back of the door panel as it was for factory original parts. If your car may be missing this part, or if you intend to make your own boor panels, you may need to make these wood strips. It is a simple part to make.

The wood strip is a truncated wedge 16 inches long, 1 inch wide, 1/2 inch thick at the wide edge and 1/4 inch thick at the narrow edge. It is to be installed with thin edge downward about 3/8 inch above the steel panel inner opening, slightly angled to be parallel to the top edge of the door, and 3 inches from the rear of the inner door shell. You can screw the wood strip to the inner door shell and lay the panel over it, or you can attach the strip first to the panel before installation as was factory original.

These pictures (compliments of The StevieG MGA) show the wooden batten behind the door trim panel. Notice dimensions for location.