On our sportfisherman we put on of those SS hooks on the outside of the door, looks like a
screen door hook. I assume the doors were similar but the door would
lift up and over the latch in decent wave action.
Of course that meant when you went forward you locked anyone into the
boat, but it was not possible to lock yourself in though.
Not a Cat of course but maybe similar doors?
On edit if you want real temporary, a wooden dowel laid into the track will keep it from opening and leave no mark