I bought this one and it was just fine:
Cast Acrylic Sheet, Clear, 12" x 12" x 0.118" Size (Pack of 8) https://www.amazon.com/dp/B009AEAFIE/ref=cm_sw_r_cp_tai_qzqQAbNMM3T8G
Of course, with Amazon sometimes you don’t get the same exact thing every time because they do that commingling thing. But what I received was nice paper-backed Chemcast. I set it to proofgrade and had no problems.
… In fact, I just ordered some more because it’s almost the same price I pay locally, without the trouble of having to lug it home on the train from Canal Plastics.
12x12x8 = 1152 sq in for $33.21 from Amazon
vs. $32 for 12x24x4 from Canal Plastics
vs. $47.50 for 12x20x5 for Proofgrade