There is a seal that is sold that actually adheres to the inside perimeter of the flap. I talked to the manufacturer of the flap but have unfortunately forgotten the name. The toilet does not have to be disassembled for its installation. You flush the system and then turn off the electrical supply while it is open and then peel off the old strip and replace with the new disc. In the interim period of time, I have been spraying the flap with silicon periodically and it seals well.
Pres