I used a circular polarizer filter which, if I oriented it to darken the sky and accentuate the clouds, the rainbow disappeared. But, with it oriented to maximize the rainbow, the rainbow was more prominent than without the filter.
My tripods were in a different car so I had to rely on hand holding which isn't really a problem there except the higher the camera is the better the scene. You can only hand hold so high, especially while bracketing for HDR processing.
Here are three shots followed by a video.
Apparently Blogger uses Flash for the videos which won't work on iOS devices. So, here's a youtube version of the video.