AL in NY -
If you plan on having the panel re-plated anyway, you could solder or braze the new piece in place, assuming you can take one off a cheap repop or a damaged original. Be sure to note its orientation before removing it, though! The placement of the notches and screw holes is critical. I wouldn't try using an epoxy to hold the shroud (or whatever the heck it's called!) against the panel's backside because the pop-out cable is under quite a bit of bending pressure as it comes out of the head. It could easily become dislodged from the head and kill the engine or even create a short behind the panel.
I have several original oval speedo instrument panels in the basement somewhere. I'll see if there is one beyond repair or is not a candidate for re-plating, yet still has the shroud. Should there be such a poor soul in my dungeon, you may have it. If that doesn't pan out, there surely must be a reader here who has a damaged panel with the shroud still in place that he'd be willing to give up.
Marshall