when you are securing the screen into the slot you are stretching it too much, resulting in a deformed shape when you get all the sides secured.
Use a piece of screen that is a couple of inches past each side of the aluminum siding, then do not stretch it too much! When you roll the liner in, it stretched out the screen. So you need to leave about 1/2 inche give so you will not get a deformed shape.
On my first try I also had a mishapen form, but reworked it till perect. I'll try and remember what else I did to correct the deformaties. Bu the main cause is you are overstretching the screen.