Building out as
@Dave T Hobbit
suggests is by far the easiest. If you want a hole, cutting part of the plastic away and building what you want with Green Stuff is easier than trying to reshape the plastic itself. What you're trying to do is probably the hardest, and while it can be done (and acetone isn't the worst choice of chemical to try it with) it's definitely in the book of Highly Advanced Techniques.
Ask not the Eldar a question, for they will give you three answers; all of which are true and horrifying to know.