Printer enclosure heater cowling

I've had real trouble with thin walls splitting apart, particularly on larger prints, (a. because they're larger, b. because the take longer to print -- i.e., overnight, and my printer is in an unheated shed). See last picture.

Figuring the ambient temperature drop at night was the main cause, I decided to get an enclosure heater. And an inexpensive temperature controller. After installing and testing the heater, I found that using as just a radiant heat source was heating the enclosure su