Metal or wood doesn't matter. Just a stud frame. How it can or should be designed in your case cannot be guessed from here.
The minimal version would be (viewed from the side standing in the attic) a triangle with a door height (~2m) at the stair landing, which slopes down to 0 at the opposite end of the stairwell.
From the triangle, you can make a rectangle for a bit more headroom when climbing the stairs, but of course, you can also extend either variant up to the ridge/rafters.
Alternatively, a hatch door might be an option. Screw or stick a frame all around the stairwell (to create a smooth surface without joints), take an OSB board (28mm) with approximately +10cm more size all around, stick insulation on it, and simply apply a seal on the frame. The weight of the board should hold the "door" as tightly as regular floor hatches do. Costs should be under €100, and the effort is minimal... provided there is no stair railing in the way ;-)
For easy opening, just attach a counterweight via a cable winch to the rafter above.