One problem with assigning a conceptually accurate sign to "plastic" is that there are different kinds of plastic.
The adjective plastic means something that is pliable, so the concept would be flexibility.
However, some plastic products are hard, which is probably why SigningSavvy uses the GLASS concept.
Then you have a plastic bag, as in "paper or plastic?" which is a soft product.
For accuracy, spelling is probably the best way to go.
Of course, if a common sign for PLASTIC is accepted and used within my local community then I would go with it.
However, if the sign is the result of SEE creativity by a hearing teacher or terp, I would be wary of it.
Just my 2 cents.