The technique I'm using is to heat a piece of perspex in the oven until it's plyable and then put it between the formers and clamp it tight until the perspex cools. As you can see this took a few goes changing the oven type from fan to conventional, the temperature and the length of time the very warped pieces are wit the oven set to conventional.  The left hand version is fairly successful and used a temparature of 180 for two minutes in a fan oven.  The perxpex is 1mm thick.  I was happy with the results even though the corners seemed to get too hot and turned white.  Fortunately the perspex was oversized so I could cut those off.