For sure no access from the quarterberth below? or from the cockpit locker opposite (under cockpit and that way??) I expect you've checked that, but just asking
Know any small people that might get around/under the cockpit?
A cutout in the side of the cockpit backrest wouldn't be horrible, and you could buy a cover plate like:
Make sure it's big enough to get your arm through.. Might even create some useful storage space.