Thinner material just means it will get to temp and lose temp faster, but the heat will still damage the seat. And in order to thermoform, you either need enough mass due to gravity and time, or (like in most cases) vacuum.
I don't want to vacuum form onto the seat, I want to make a mold of the seat which will be used for vacuum forming a thicker piece of plastic.
I was thinking that, with a heat gun and a thin plastic that could get to temp and cool easily while I form it by hand, I could make an inverted form of the seat from which I could make a heavier duty mold for proper thermoforming.