It is very tough,but it can be done.Helps to do it in the middle of the summer on a really hot day,as the poly is more flexible.A few extra hands and some large "c" clamps would help also.Wait till summer if you can.
iv'e done it in a heated garage,you will need a torpedo style heater to blow warm air across they poly to keep it real flexible,and a couple of friends to help you out...