this, but don't do glass, do lexan or similar-

I have steel casements with historic glass, and they are so beautiful you want to cry. right up to the winter when they radiate cold like nothing else, or summer when they become a rotisserie rack, and you want to cry from sheer discomfort.

glass needs a much thicker frame than lexan and is much heavier and harder to wrangle, you can even skip the frame entirely and get a piece that covers the opening +1/2" - get a shop to pre-drill small corner holes and face mount it with screws and small clear washers, just be careful not to over tighten and crack the plastic.

the real problem with inserts (beyond weight for large windows) is how out of square old windows often are, coupled with cranks that often don't leave enough workable depth in the frame to make a reliable seal, so face-mounting is a good work around while you figure out if you need custom inserts