a large circular stone could be set into the rectangular opening, here's an example https://www.amazon.com/Luxurious-Engagement-Zirconia-Sterling-Eternity/dp/B0B5HD6TXV

Or you could get small stones and fill in with a "grid" of small stones like this https://www.jewelexcess.com/products/1-00-carat-white-diamond-ring-in-sterling-silver

or who knows if you're a search ninja you might be able to find a rectangular stone the perfect shape, but that's less likely.