It depends on what you mean by sealing off. I closed up all the holes in my box with liquid aluminum and then coated the inside of the box with liquid rubber. Then I put silicon where the wires come in and carpet on the bottom of a custom made lid. I can still get access to the dual optimas easily but the noise is cut. If you mean seal as in you can never get back in there, it is not a good idea as every battery needs occasional maintenance.
2009 King of the Hammers
BUCK Wild Racing
Co-driver: Pat Quirk
Rover Tracks www.rovertracks.com
DJ Safety www.djsafety.com
West Coast Rovers www.westcoastrovers.com
Reel Driveline www.reeldriveline.com
Made it further than half of the other guys, but the Hammers won.