It's a pity avp classic has no way of force loading a custom single player map. Backing up and replacing the 1st marine level is hacky but it's the only way.
Maybe I could have called it 'Derelict.rif' in the zip file to save the renaming step, but I just figured if I'm asking people people to overwrite original game files, I want to be very sure they don't do it by accident! Hope you enjoy the map.