Take your time with the chopsaw, and keep nipping until you get just the tightness that you want.
Put a bend in the 1x as you move it into place and then compression will take over when you let go.
Put the pre-drilled-to-the-cedar, plastic rack on the 1x after the 1x is in place.

Good luck.
Take pics.

Edit:
If you have metal framed windows it may be possible to use a piece of light aluminum angle (hardware store) that borrows some existing frame screws to join the 1x to the frame. It could be made tidy and would positively secure the 1x if compression didn't work.