If you have access to electricity in the booth, you could make one of these fairly cheaply. Just freeze gallon jugs of water for the "ice blocks". And use 4 inch PVC elbows for the out pipes-two is better than one. I have a family member that routinely does this for her booth at craft fairs.
http://www.ehow.com/how_7714669_make-air-conditioner-styrofoam-cooler.html
http://www.ehow.com/how_7714669_make-air-conditioner-styrofoam-cooler.html