IMO incorporating a water chiller into the test bench to maintain constant temperature is a must. Even using a huge reservoir (several 100 gallons) wouldn't hold temps for long term testing and I doubt creating a workable fan/radiator setup would be practical (size, response time and ultimately cost).
Refrigerated Circulating Water Baths are readily available but expensive. This could be the single most expensive piece of equipment if purchased new. Expect to pay $2,000 ~ $5,000 US.
http://www.vwrsp.com (VWR Scientific Products)
http://coleparmer.com (Cole-Parmer)
VWR and Cole-Parmer both sell PolyScience chillers under their own name. Other popular brands are Haake, Lauda, and Julabo. I personally use VWR and Haake chillers in the lab where I work. Haake chillers have an excellent reputation but are also more expensive.
Most lab chillers incorporate a refrigeration system and PID control, while a few use TECs. I would highly recommend the refer units.
Most chillers are available with either analog or digital controllers. The analog units are less expensive but typically can only maintain a +/- 0.2 temp. Some of the better digital controls claim +/-0.01 temps.
Even the small lab chillers can handle around 200 watts~300 watts @ 20 deg. C.
Check the sources above and then keep and eye on E-bay...
