Just looked at mine, and it's just the wedge shape you see in the pictures you posted. The metal tab pushes it up against the upper part of the opening in the dsah, providing some friction to keep it from opening on it's own. The front edge of the plastic hits the tab sticking down in the dash to limit how far it opens. If you feel underneath, it's extruded through the holes in the metal tab to keep it in place. I'd bet you could shape a piece of plastic or wood to the right size and epoxy it onto the metal arm.

 

It felt like the plastic piece had two protrusions that stuck through the holes and were melted flat to hold it in place.

 

Here's a few pics from my '88. The metal bracket should slide out out. There's a small tab on it that clips into the slot where the plastic "ramp" fits.