Following this thread because I'm in somewhat of the same position as you. I have PVC enclosure in a finished basement (600sq ft) but I have a wall mounted heating system that I've been running at 79 degrees to maintain the ambients. I'm fearful that once it gets colder outside, I may have to turn it up or worse yet it could stop functioning if it gets too cold outside. Do you have any means for heating the basement?

You mentioned an RHP..I've been told these work wonders for PVC enclosures. I've considered getting one but I was 'hopefully' saving it for a time when I need to upgrade my enclosure. I don't want to fuss w/ trying to install it now and messing w/ temps, etc.... This may be a good option for you. I would suggest calling BOB at Pro Products to discuss your options.