There's a reason the U.S. military trains in body armor. I had to essentially live in mine for a month in California for training one summer. The following spring, I was in Iraq and glad for the training. It kept me from having problems when I went from 65 degrees to 110 degrees. We still had to wear our gear nearly continuously. Mine clocked in around 50 lbs. with ammo, plates, and water.
The fans are a good idea. Try sourcing one for paintball as they blow air directly on the lens. There is a company that uses a pc fan and a hose to remote mount the fan and better direct the air. Be aware that this may cause problems for those who wear contacts, prescription or costume.