Cabinet temps getting low

What are some ideas for raising the temps inside a cabinet?

I have a 400 hps, and 4 26 watt flouro's, and a heating mat currently trying to heat a 3' x 1.5' x 6' cabinet.

Temps in the middle of flower are only getting up to 65 degrees and as low as 55 during lights out. (outside the cabinet the temps range from 50-60 degrees.)

My growth has come to a stand still - with only 4 weeks flowering left.

I have lowered my exhaust to the lowest setting on the fan.

I have the same problem every winter, just not as chilly as you..these are some of the ideas I have had or things I have done to raise temps w/o burningthrough my heating oil..

..reroute your scrubbed exhaust back into the room
..electric heater
..un-remote the ballast (place it in the flower room if it's already not)
..a dehumiddy will raise temps, but lower your RH
..what about a non vented 150 hps from home depot? you could always just aim it at the girls, and I bet it would raise temps just plenty..
good luck..
~cd

edit:thats a pretty small space to heat m'man..should be relatively easy to heat 'er up..

Low temps are not something I thought I would ever have to worry about with the 400hps.

The cabinet has a veg chamber below that takes up about 20-25% of the space. The temps are about 5 degrees lower as you would expect. I would like to add the heat to the veg chamber so it could rise and help the flower.

I thought about bringing in the ballast, but it is a wet environment so prob not a good idea.
I am leaning towards wrapping insulation around the cabinet or a heating blanket. The drawbacks are that I lose my "stealth" by doing so..

Check out some of the small propane heaters, maybe you could squeeze one in there for lights on, and use an electric heater for lights off? That's kinda what I'm doing, but my room leaks like a sieve so I can't get it above 62 right now. 49*F lights off, plants are sad now