Creating a complex custom shape

Create a rectangle using rectangle shape tool.

Press Ctrl+T. Right Click and select Warp. Select "Flag" from the preset warp shapes. Adjust settings as shown.

Press Ctrl+T. Right click and select Flip Vertical and place on the other side.

Duplicate the path. Press Ctrl+T. Right click and select Flip Horizontal and place on the other side.

Similarly duplicate the path several times, scale down and place as shown.

Select and duplicate all the paths. Press Ctrl+T. Right click and select Flip Vertical and place on the other side.