Filling door gaps, 4 dr to 2 dr conversions

I've asked this elsewhere several times but have never received a reply.

Articles covering the conversion of a 4 dr vehicle to a 2 dr (example Tex Smith's soft back book) give a good idea as to what to do into the point that the rear door sheet metal is attached to the car. They then jump to the finishing stages without explaining HOW to fill the gaps between the ex-door panels and rear quarter panels.

You can't pull the skin rearward as that would increase the width of the gap at the back of the front door. So was "gap rod" (thick welding rod) used?