I have a window with 6 panes. I made 6 window plugs that have handles and a snug fit. You can see them here:
They are a square of MDF. Then with some small pine risers, 20cm x 5cm and 5cm 'tall' to screw the handles onto, glued and screwed somewhere near the middle. Then a square of super dense 5cm thick rockwool to fit the MDF, but with two 20cm x 5cm square holes cut in it to fit the handle risers (slides down around the risers). Then that whole thing wrapped like a present in fabric and hot glue gun and stapled to the back (only ugly for those outside the building!). Then with the fabric nice and taught, handles are mounted where the wooden risers are, just screwed through the fabric (open a hole inbetween the weave first to avoid laddering the fabric) and into the wood risers.
The work a treat and can easily be fitted and removed by hand. They even make a little airgap between the window pane and the MDF backing. They end up quite heavy with the rockwool, so if you have a bigger window, you might want two sets of handles on it for two people to lift and fit.