The tin of the muffin is essential to its recognizability, and does a lot of work in covering up the internal workings, as well as holding up the top.

Links:
https://3dwarehouse.sketchup.com/model/32daa99d15ea1a04946a13002971c561/Muffin-Car-Cupcake-Car-Tin-Bending-Template

As is mentioned in the sketchup, the tin is meant to be made of 26ga or 28ga galvanized steel. More recent models have successfully used 30ga steel to reduce weight without sacrificing too much integrity.