Wall Hanging Project I Am Having Problems With

I am trying to make my first quilted wall hanging.

Here is a pic of the front:



And here is a pic of the back:



The two pics do not show the completed quilting, but the quilting is now finished.

The quilted fabric panel is not exactly square. It is supposed to be. It was out of shape when I started, so I ironed it as straight as I could get it. Last night I marked a board to the shape the panel should be, then pinned the panel on the board and let it set all night. When I unpinned it from the board this morning, it went right back to not being in shape.

My next step would be to add the three pieces of fabric at the top that would hold the dowel, and then to sew on fabric for the back of the wall hanging, possibly with piping all the way around the edge.

I am stumped as how to proceed. I don't want to make a crooked wall hanging :-0