The last time I had some raised-bed gardens I extended the growing season by building a tent over them. I just put a row of pvc pipe pieces in half circles by sticking the ends in the dirt; the sides of the bed anchored them. Then I laid a plastic drop cloth over them and clipped the plastic to the pipes. It took all of 15 minutes to set up. It stayed very warm inside, even though the ends were only nominally closed. I had lettuce and spinach growing all winter here in Seattle. I doubt you'll get things growing in the middle of your winter in Alaska, but you could probably add a couple months.