I think the key part of this entire endeavor that has not been discussed is not the panel but the electronics. Even if the glass panel might be able to handle the temperature swings, that does not mean anything if the morning dew or frost short out the electronics. (The humidity ratings in Randy's post are extremely relevant.) A regular consumer grade plasma was not meant, nor designed, to be in that operating environment. Additionally, by being outdoors you will expose the unit to greater temperature swings between on/off cycles and the repeated thermal expansion and contraction will greatly increase the chances of other circuit board chicanery.
Anything you save up front will inevitably be offset by the manufacturer denying warranty coverage due to improper use. Do yourself a big favor and just spend the extra money on a product like Eric has pointed you towards. These products have the additional cost because it costs money to properly prepare electronics equipment for outdoor use.
P.S. I've personally experienced a circuit board that had a broken trace and would only work when the temperature of the board was high enough to let the trace expand and close the break. Since then, the elimination of lead in solder and the general miniaturization of electronics has only made these circuits more fragile outside their intended operating environments.