Awesome idea for later:
Cast a 3” or larger tap to cut large threads into window sill. Make same size threaded wooden plug that can screw in tightly. Remove plug to check on insulation in the sill. Alternatively, could hold a wooden block in place that makes a perfect fit in a square hole and use embedded magnets to pull it out.
This can all be done later though. We could start by leaving it sealed up and drilling/cutting a hole in it later to check on things.