I've stacked supers up on top of an empty hive box that had a 40w lite bulb in it. Had a blanket over the top of the stack. It warmed up the frames in the stack well enough to extract easily. I kept a watch on it to monitor the temps, didn't want the wax melting.
I've now got an extra fridge. I was seriously thinking of disconnecting the freon portion of the thing, using a 100w lite for the fridge lite, installing a small 110v box fan and a temp control switch from home depot made for free standing heaters. I'm thinking I could keep it between 95 - 100 degrees as a honey warmer, could do it by the bucket, or keep it at 85-90 for warming frames for extracting. Knowing me though, you'll get one going before I do!