You will struggle using a coil element in this type of bender. The smaller line bender I made used the coil element from a bar heater with the ceramic tube so sagging was not an issue.
Take care to heat slowly when line bending polycarbonate as it retains moisture from the manufacturing process and it will boil causing bubbles. If you are looking to fold polycarbonate, not acrylic, you can use a sheet metal break.