In Jerry Walker's videos, he concluded one of the most significant issue was that the back fan opening allowed cold air in during heating and sucked too much heat out during cooling. He suggested partially closing the back fan entrance. In the image below, you can see he has partially closed his off with Kapton tape.
2020 New version
In the updated version, they have added louvers to the back fan opening. This provides and even better solution by nearly completely closing off the opening during heating and partially restricting the air flow during cooling. The louvers are one big reason to make sure you look for the new version when you go to purchase your oven.