My existing brick to brick width opening size is 1620mm. Height 1090mm. Wickes French doors are 1490mm x 1090mm. Allowing 5mm each side to the wickes door frame = 1500mm. Therefore 1620mm less 1500mm = 120mm divide 2 = 60mm short on both sides. My question being, can I fill this with a 60mm timber batten/frame screwed to the plastic frame, then long bolts through both to brickwork, cover it with plastic trim on completion to hide. Or is there plastic frame packers or should I have the doors and frame made to measure. Obviously the cill will be too short and may start looking rather messy. Any help much appreciated.