I decided to try adding a border around the old smaller transom window. I made a piece of backer glass that fits the space and centered the old transom insert on it, then cut some matching pink and green glass to fit around the edge. The below photo is before I glued the border glass down.
Unfortunately I just realized that I forgot to glue the backer glass to the old transom, so there will be some tricky adjustments once the border glass adhesive cures. With three layers of glass, it’s pretty hefty.