Thanks BB, George, Jim, MrRick!
With my wife having dabbled in the glass realm, I've learned it is never flat and never square. This one was a bit beefier than most with a thick copper wire running the perimeter and the edge foil being thick. I left a generous 1/8" gap all the way around (1/4" overlap) so it wasn't too far out, but still a tad wonky.
At first I planned on space ball type spacers to keep it centered and then some caulking to keep it all in place. GLad I went with the washers since it is easily removable and since they are sunk in a bit below the borders thickness, they can put a variable amount of pressure on the edging and wedge the panel in place.
Next is to see if she can get it home and inside while in one piece. She had it sammiched between two 1x10 flat pine boards with foam when bringing it over, good protection, but this is now about 14" wide so the frame is exposed to scuffing. I'll have to figure out some cardboard edging to protect from that.