Seems there's only one photo ever of this backglass (at the IPDB), which may not even exist any more, so I figured I'd do a hi-def (2000x1752) rebuild. All the main issues have now been fixed, altho I cheated slightly and left some edge-shadowing intact across a couple small areas that didn't seem to mind it.
If anyone wants the PNG file to make this even more perfect, let me know. --Ike