I apologize if I don't get all the terms quite right here. Hopefully I'll be clear enough someone can help.

About 10 or so years ago, we had the double sash windows in our Arts & Crafts bungalow replaced with more energy efficient windows. At the time I was expecting them to be just like our old windows so I was disappointed when they were installed and the upper sash was just one pane rather than 3 panes with mullions. Our contractor felt bad that this hadn't been a topic of conversation and quickly offered to make fake mullions to clip over the full panes for the same general look.

He was a wonderful contractor who has since died, who did MANY good things for our home, this one being the only one that wasn't great. Besides not liking the block-y shape (the originals had more detail in the cut), the clips don't stay in well and the fake mullions fall out all too often.

Would anyone have a suggestion for a better solution? I don't want to go through the expense of a another set of replacement windows.

I'll attach a photo of a mullion that fell out this morning and a clip. I'm have to search for the other clip which must have fallen into a stack of stuff below that window.

Thanks.