Thanks for the reply guys!
The problem is that I don't own the machinery so trial and error will be time and money consuming.
CoryPad - I read that for Ultrasonic, the melting temperature difference (I guess it meant Tg) of the polymers to be joined have to be <20 degrees Celsius (between ABS and PET its 40oC). Also in tables I found of Ultrasonic welding, it appear as not weldable together.
Do you have any positive experience doing it with ultrasonic?
Also, since the PET sheet is very thin, I'm afraid hot air will practically tear the sheet
Your advise is mostly appreciated