Yea, I second what Polo wrote. The battery can be covered, but you need to provide some sort of method for it to vent. Preferably, it should vent to the exterior of the car and not into the interior of the car.
you could get a little computer type of fan and mount it plush on the outside of the box facing in so that it blows air through a hole cut for it that would give it ventalation enough maybe a small port at the bottom for the air to be pushed through by the fan but just an idea i duno if it would work though it sounds cool
well if you get a sealed battery such as an Optima Yellow Top, sealing should be no problem, as its sealed already. but if you go with a common unsealed battery, then you'll definally have to have ventallation.
Does Optima ever gas? When used with a properly regulated constant voltage charging system (such as an alternator) the Optima will usually not emit hydrogen gas. However, gassing can occur when charging at higher voltage levels or in extreme high temperature conditions. In automotive applications this typically will not happen if the alternator/regulator stay below 15 volts.
Like people all batteries emit gas just some more than others, lol. Polo.