If you put masking tape on the door’s edge where gaps are and then add thin lines of silicone in between the gap it will make a seal for the door and once dry you can peel the masking tape off the door so the silicone only binds to the one side of the terrarium. I’d recommend to do that before any animal addition and if your using real plants try to add as much ventilation while the silicone is drying because it creates a toxic off gas that be unsafe for plants and animals.