You can try punching a disk out of clear material, like the stuff used in packaging all the time. A standard hole punch happens to fit Autocord and Rollei counter windows, for example. Maybe if you hammer the punch over a soft material like a heavy mat it will take on a lens shape? White glue, Elmers, applied with a toothpick around the rim of the opening in a very small bead will not fog the material like aa super glue would do.
These windows pretty much always are inserted from the inside, by the way. The original magnifier might be inside the top?