I have yet to build my rubbermaid box BUT I believe you are using the bottom of the rubbermaid which encloses the box, don't use the bottom. You can see in some of Bodoms pictures, he's not using the bottom until, either it starts stinking or most likely when you have to 12/12 due to keeping out light.

Prop your stuff up with CD cases or books like in Bodoms pictures and it basically keeps the whole bottom as a passive intake. It should help your temps while vegging and all.