Thank you, Grant. Multiplying skills are common on this forum!
Much appreciated Christian. I was expecting you to be the one who pointed out I was building the mirror image of TFFM. Not exactly sure how that happened but now I'm committed
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The first version or the fifth?
My solution was to glue a tiny rectangle of wood (the precise size of the interior of the frames) down to a piece of cardstock. I then built the perimeter frame around that rectangle of wood (glueing the corners). Then I added the mica glass (glued with CA to the perimeter frame). Now I could remove the window panel and insert the mullions. Like most everything, it was easy once I figured out how to make it easy.