My new home has a ply push up loft hatch that is supported by the
architrave around the opening. It is always a pain to open. I saw
those plastic prefabricated drop down ones in B&Q and elsewhere but
the standard width required seems to be 56 cm. My joist to joist
opening is a tantalising close 55 cm.

How easy do you think it would be to chisel or otherwise cut back half
a cm on each joist to a depth of a few cm to accommodate it.

Or should I just make a new one from ply or mdf? In fact thinking
about it, maybe I should just add a hinge and some sort of catch to
the existing one and replace the architrave to open it up. I guess I
would need some sort of insulated flange to close the door up against.