Duplo Universal Seat Adapter & Child Seat

My goal was to make a Duplo adapter to enable old small Duplo children (early 90s?) to sit in cars, on modern chairs and everywhere else where there is no big center stud. This was accomplished with the child seat.

Then I improved it further by adding holes in the bottom for both standard and big center studs (inspiration taken from the classic Duplo chicken) as well as making the top stud work as both standard and big center stud (inspiration taken from the classic Duplo tree part with two b