Here's what I'm going to do: buy the thickest rigid pink styrofoam insulation Home Depot carries (like that stuff they put on houses before aluminum siding goes up), and build a little box.
I'm lucky and have an a/c vent right by where I ferment, so the plan is to have a corner over the vent to pump the air in, and then the insulation will act as a mini-fridge for the fermentation days.
So far using thinner rigid foam I already had on hand, I build a 'lean to' over my fermenter with a thick towel for teh roof, and dropped the temp about 5 degrees F.
I figure with a more sealed box shape, with some vent holes on top (since heat rises) I can probably maintain pretty good ale brewing temps.
a 4'x8' sheet of taht thick foam is about $21.