HI jonnie
looks like you may have done a lot of work to achieve the already achievable.
If i understand correctly.....

Cut a hole in the flat roof/ceiling that you can move and resize
link

Ability to change the color/texture of the hole sides.....

Add a hatch from standard library to project....export it... import..
flip to horizontal and check the box for stairs... elevate into your ceiling,

That may have done for you.

Also, approx 2 yrs ago one of our Russian friends shared a file in which there were about a dozen different invisible shapes, not sure how or where you would search for it, but its in the ether somewhere.

Mike