Fitting loft hatch

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.

Not very. Besides which, it may weaken the joists. Far better to make the lid fit the hole!

Can't you use the existing one, maybe cut down a bit - and add a light softwood frame so that you can hinge it easily. Suitable hinges and catches usually come with loft ladders - but you can probably buy them on their own.

The idea is that it hinges downwards. You push gently upwards on the side opposite the hinges, and that releases the catch. You can put a slab of insulation on the top side of the door if you wish. You may be able to put some light battens - and strips of draft proofing inside to opening for it close against - but bear in mind that there needs to be a small amount of up and down movement in the closed position to allow the catch to operate.