I'm sure that someone here on the Forum will have some "trouble shooting" tips for you on this problem. But, I can tell you that Oliver spent several years trying to find a way to install something other than the "simple" microwave. As I understood the main issue during the time they were trying, heat dissipation was the main issue they were dealing with. Therefore, it seems (to me) likely that you have basically honed in on the root cause of your problem.
Good luck in fixing what you have.
Bill