John Rumm was thinking very hard :
Perhaps I need to modify the deck itself to make a hinged edge section, so
you can open that, and then fit the mulching plug to block the ducted exit.

They used to sell such a mulching adaptor for my model, no longer
available, so I have had it in mind to make such a plug for a while. I
just need a nice day and the will, to get a roundtuit.

I have in mind just a sheet of thin steel cut to fill the gap, which
bolts into place. It doesn't need to be the same thickness (5mm) as
steel deck.