When dealing with Lead Acid & Sealed Lead Acid Batteries it is a common and potentially dangerous assumption to think that they can be totally enclosed. Even Sealed Lead Acid Batteries need to Vent and give off gases under certain Circumstances. So it is important that ventilation is always considered. There is also the issue of heat dissipation which may require consideration. As little as 4% Hydrogen in air can form as explosive gas and hydrogen is a light gas which means it can form pockets at high enclosed points. It will be best to consult the manufacturers if unsure about a battery installation or if there is a special application.
Consider the venting needs of lead acid batteries when selecting your enclosure.