My canvas is not square ... what to do?

I just finished up a 16 x 20 acrylic painting that I wanted to give to someone as a Christmas gift. I went out and bought a frame for it and everything but when i went to place the painting in, I realized the canvas was not completely square. I can force it in there, but it causes the painting to ripple a bit. I don't know what else to do other than to brace the back with wooden triangles to make sure the canvas frame is square and restretch the canvas. Is this safe to do? I'm worried my painting will crack or end up looking warped or something. Please help!