Gettingto the 2nd battery compartment under the sole

1984 JJT #313...I have a first starter battery under the sole, and a second systems battery under the starboard settee.  There is an already installed battery compartment right behind the starter battery, another foot closer tot he engine, but it appears fully glassed in from the top (no access panel like the first), and there are two knees/braces designed to keep the pair in place between the first and the second (now empty) battery compartment.

I'd love to get back the sotrage room and the better placement/security of course by using the second under sole battery compartment, but cannot seem to get a battery back into it over the knees.  The entire install appears to be original (the dual compartments, the glass work over the second, the knees between), so it would seem that it was built with this peculiarity, and likely why the prior owner just put the second under the starboard settee.  But it doesn't seem possible that it is a engineering/construction oversight.  Anyone else have this issue, and if so, what'd you do?