I originally tried to find a sealed battery box. Only ones I could find were for drag racing, made of alloy and about $300 ea.
Why not use a sealed battery in a non-sealed box with the vent tube going out through the floor like BMW's and Merc's. DIN55 or DIN66.
http://classic-ford.org/forum/topic.asp?ARCHIVE=true&TOPIC_ID=15367
Mine is done like this and have had no problems and would pass a RWC in Vic.
Craig.