I have actually done just that.  I cut the front of the compartment open
with my Dremel tool.  I had the intention of putting my 6-disk CD changer
in there.  After I cut it open I found that it wouldn't fit.  The latch for
the swivel on the front seat (Westfalia) drops down and the the opening
wasn't large enough.  I completely removed the metal and now the carpet
just covers the hole.  The carpet is stiff enough to keep it covered and
works well enough.  I would like to add a door.  I keep my wallet and radio
face in there when I park the car.  The carpet hides the location and I
don't think that any theif would think to look there.  As for leaks, I have
no problems.  I don't have a battery in the second battery location.
That's where my CD changer sits right now.

Hope this is useful.  You're right there is a lot of room under the seat.