So, as you can see in the earlier photographs, there was one corner that was cracked off and it was a pretty decent size and it was extremely warped. First thing I did was place down on a piece of paper, held it flat and traced it. At that point I had the original size. I had feared that the heat would potentially make the plastic shrink, which would be a complete disaster. I kept putting this piece back in the oven for about 10 minutes, which was long enough for the plastic to reach the temperature of the oven. Then I'd take it out and I'd lay it on the piece of paper and sandwich the piece between the counter top and a cutting board, putting my weight on it. The temperature would drop quickly and then I'd check the piece for size against the outline I took, and deformity. The size never changed, which was great. But it was still warped for many iterations. I kept doing this, over and over, while increasing the temperature until the piece was relatively flat. Once I determined that 300 was the temperature, I threw the whole thing in, left it for 10 minutes and then attempted flattening. It wasn't perfect, but worked well enough that it no longer looks deformed. All of the nail and pin holes were completely flat.
If I was to do it over again, I'd probably keep going higher and use two pieces of glass to sandwich the backflash. I'd place a 30# weight on the glass once I pulled it from the oven and then let it cool slower between the panes of glass. I'd suspect that it would flatten quite well in that scenario.
At one point I couldn't find my corner piece. It had fallen off of the rack in the oven and was sitting against the back wall, touching the bottom of the oven. I'm not sure how it didn't completely melt. But that could have been disastrous.