3x3 doorway, closing flush with the wall, and all the redstone operating it is 3 high. There are 4 entities in the door that act as long delay timers, and there is one more to limit the delay of one of the timers. Lots of observers and hopper timers, and even a block or 2 in the doorway, which are entirely legal by the "official" rules for redstone doors.
There is a lever on the left side of the machine, on the side of a note block. Flick that and the door will open/close. wait for the door to stop making noise to power again. There are brief 10s pauses during the operation of the door, so please wait for that too!
P.S. Don't nudge the carts or touch string/pressure plates, thanks!