Modification and installation of cabin heat duct

The KAI plans call for M3 rivnuts to provide attachment points for the cabin heat flap valve. M3 has been abandoned everywhere else, replaced with M4 and this will be done here. Four of the six rivnut locations are shared with the cabin heat duct. The flange width is already marginal for M3 rivnuts, but totally useless for M4 rivnuts. So doubler flanges were created that are rivetted to the body of this duct, while providing enough material for the use of M4 rivnuts. This doubler was made from 0.8mm sheet