bit of a bummer about the heater bro. But good that the guy has given you a refund
I got one too yesterday (refund not heater), I ordered a thermometer just before setting up my flower cab, 15th Nov (Seems like longer than that don't it). For that amount of time and the sake of 2 quid I may as well have gone to homebase and got one :-S
I gotta sort summin out for my drying too. Got a while to go before I start considering this but still have to. I did ponder similar to yours, a card box at the roof of my flower cab, connected to my exhaust fan so it acts as a kind of duct. I rather think that the humidity in there may be slightly too high for it though, which will make it take too long to dry. I guess it's one thing we'll have to see eh...
Similarly, I shouldn't be bringing that shit into my house... Although, I do have a few sneaky and rarely used cupboards