Get the Fly High Pro-X Series Fat Seat Bottom
It fits like a glove.... I don't know the dimensions but they are honestly made for each other.
This picture shows the size, kind of.... it clearly isn't full, or pushed up all the way... we just run a ton of bow weight so we have it smashed at the back of the locker and don't fill it full.... but you can get the idea of how well it will fit --- and when in the right place it will fill to about 95%.... the only reason not 100% is because the latch for the locker sticks down and I don't want to risk poking a hole