Can I block a skylight during summer without condensation?
If you do find condensation then you have a more significant problem that you would want to repair.  Here's why:  The drywall tunnel, being exposed to sunlight should always be hotter than your house, which means that relative humidity will be lower than in your house even if infiltrated with air from your house.  That also means that any condensation would only be against the insulation barrier at the bottom.  Unless you have other moisture infiltration into the tunnel that shouldn't happen.  If it does then you need to insulate the tunnel from inside the attic or else find the leak that's letting moisture into the tunnel.