Synergy came with a 50 ltr bladder type holding tank. Frankly it smelled really awful even when empty. Early on, I replaced all the hoses with higher quality hose and wrapped the tank in a heavy duty trash bag and then put a heavy duty polyethlene bag over the trashbag. The smell lingered when I opened the locker and so I scraped and painted the interior of the locker. And with that, the smell was minimized such that with normal ventilation, the smell was not apparent.
I always saw those measures as temporary at best and planned to install a proper polyethlene rigid holding tank. That process began today when I pulled the old bladder out of the boat and began measuring the lockers near the head so I could figure out the tank sizes that would fit and order a new tank. The issue will be getting the biggest tank that will actually fit through the opening and into the locker, and develop rough dinmensions to begin to engineer a 'foundation' for that holding tank.