Krang´s giant knucklehead, based on the great episode "Casey Jones - Outlaw Hero" is - as usually - made of cardboard, plastic, polystyrene foam, modeling clay and some parts of a ballpoint pen for the front cannon. The hands / feet are cut off the original Playmates Knucklehead I bought some years ago. These hands / feet are the only parts useable from the original knucklehead.
The Knucklehead is cca 7" long (18 cm), the height is cca 7" (18 cm).
The front part of the main body is made - cut off - of polystyrene foam, covered with modeling clay and glued to the main body. The main body is made of cardboard and plastic. I used transparent plastic for the cockpit - the rear part from headphones package.
I had to make some measuring based on the screencaps from the episode, but almost in every picture the knucklehead looks a bit different. So I took something like mix from the pictures as a base when creating the parts of the model to have it as screen accurate as possible.
Krang is made of fast drying modeling clay and glued to the seat.
The last picture shows, as always, the construction of Krang´s giant Knucklehead.