I recently had to replace the main head anti siphon valve and quickly found that it was easier said than done. With limited access behind the nav station panel,I decided to go through the cabinet in the main head and cut an access door. I located where I thought the best location and cut a 6x 7 hole. Attached are three photos to explain what was done. This gave me complete access to the valve as well as the holding tank vent and shower sump exit. Once completed I re-attached the cutout by using two aluminum 1 rails to support the access door. I hope this will help the next guy that needs to do this job.