The am cuff is typically made of PLA so the part can be molded with hot water. It is molded over plastic from with the approximate dimension of the recipients arm or just warped around a 2x4 to create a U shape. PETG is not going to work well for thermoforming as it needs a lot of heat to soften. This is why PETG is suggested for making parts that will sit out in the sun. If you want to try, do some google searches for "Thermoforming PETG" and see what is required heat wise.