Tissue, dampened, and pushed into the area. Cover with tape if needed. Once sprayed, the tissue will harden, and can be removed later. There may be some retouching required, but easier than having to repaint the whole area.
Alternatively, trace the outline of the gear doors ( or any other hatch, opening etc) onto paper or thin card, cur out, and fit to the wells, securing with PVA.
Spray, and remove these masks when required, and minimum re-touching if needed.