Using shed to finish my outdoor

I built a shed 5 years ago and I grew in it for a couple years, however I had a dual port ac exchanging the air. So now I have two very large outdoor plants that are nowhere near ready and have to be brought in so my plan is to use the shed, only thing is is the ac is gone now, there is still two 6" holes about 3' off the floor, I also have a window that opens up from the bottom and slides up. So what I'm wondering is what is the best way to keep them warm while still exchanging the air? Is leaving the holes open and the window cracked good enough? I was thinking of putting a two way window fan in but the shed is only 8'×8' wouldn't that freeze them out? I have a large electric oil radiator that I can put anywhere it is more than enough to heat the small space. Any ideas and suggestions would be greatly appreciated.

Had a bit of an issue almost fried my plants yesterday even on minimum that oil heater gets too hot. So I'm gonna have to let them go without heat at night. Heres a couple pics you can't see it very well with the hps but they're turning purple from the cold.