before you get nutty with spray foam and extruded PS foam board. you may want to at least consider just building a heat exchanger. a small pump and radiator would work well given that you are just looking to drop the temp like 5 degrees correct?
you might even get away by using a very large camping cooler... if not you are simply building a closed up box.
look into pc water cooling, and those aftermarket oil radiators for peoples fancy cars etc.
something with a large area and largish dia. tubing. you are not trying to exchange buckets of energy here... just enough to maintain that small temperature drop... so a nutty radiator with a superdense fin assembly and monster fan is not necessary.
edit.
you could even build a "cold finger" type heat exchanger that would sit down inside your fermentation vessel. this would mean you need to cut another hole into your brew bucket... in the case of a carboy, you need to build a custom stopper out of somewhat pliable silicone. it wouldn't be hard to do, but not idiot proof.
here i found this on amazon. this is what i had in mind.
http://www.amazon.com/Derale-13212-Power-Steering-Cooler/dp/B004XONO8E/ref=pd_sbs_auto_7