You could always removed the whole piece of glass and replace with an insulated panel. That is what I did with the lower glass panels of our front door
You can get stepped double glazed units, so one piece of glass is larger than the other giving you a flange all around, this would fit in the position of the old glass, you would need to make a trim to finish it off though
there is a sort of double glazing film that you stick to the frame with double sided tape & shrink it with a hair dryer. My son used it, I’ll see if he can recall what it’s called, he is on holiday at the moment, but I’ll see if I can text him.