Good question. I have had to fiddle with the glovebox in my 250 SL for some time. For me, it was just a matter of patience messing with the fitment screws to the dash, and the screws that hold the door in place. It still is not perfect, but much closer than what it was. The curved arm is also not quite right, and I was never able to get the little aluminum grommet you should see when the door is open (that protects the opening where the curved arm swings) to stay in place.
I shaved the liner to make the fitting closer. That is, the leather was fitted around the flange on each side, and I trimmed it so it was only on the front, and used adhesive to keep it in place. Each little step helped with the fitting.