Yes, I agree that "fire doors could be made to *look like* ledge and brace"
but that was not the question.

You can take a standard, half-hour fire resistant door and 'face' it with
T&G boards suitably treated with a fire resistant coating such as Albi (a
rather naughty coating containing arsenic and a multitude of other
chemicals) and fit this into a rather hefty door frame (not lining) with
25x25mm planted and screwed door stops - just don't drop it on your toes
when fitting the beast! :-)

Brian G

I would be very careful when altering a fire door. Any DIY changes
could invalidate it's certification. Simple things like screws into
the door can transmit heat and fire into the structure of the door.
Fire doors are made to a specified design that's then tested. The
manufacturer supplies detailed fitting instructions that must be
adhered to in order to ensure the door performs as it should.

John