I thought an additional drop in panel would do the trick, but this would involve storing yet another item when not in use.
However, it turns out that the standard AutoSleeper table is the perfect drop in size.
All that was needed is a couple of brackets, one to the pull out bed unit arrowed below:-
Carefully sited that it fits in the gaps between the rail in the (removed) guard. A longer bracket can be fitted just below the front cupboard (arrowed):-
Then the table can be dropped in ( once you're in the bunk ........ )
You'll need a matching cushion as deep as the existing mattress to level everything out.
You might spot a little individual bunk light fitted , it can be a bit dark otherwise.
Not for the claustrophobic though.