Just tried the same thing and came up with 17.5-18in. Obviously if she was pushed through the hatch there would be some scraping and DNA residue. Have to also think that prints would be on the sides of the tank if trying to push a body through that small of an opening. She was about my size and my hips aren't very wide but I know for certain I could not easily fit through a 18inch wide opening without some maneuvering/ pushing. Even with rain you can lift a print..