Typically I use a heat gun. When I had to bend a long length of plexiglass, I used a broom handle cut to the length of the piece and my heat gun set on high. I moved the gun back and forth along the bend point and let gravity pull the end of plexi down while I was heating it. Be sure to clamp the plexi/plastic to the broom handle or whatever you use so it doesn't move on you.
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