I have been using "sportsman blankets" I bought from the site that rhymes with bamazon. They are pretty inexpensive.
I put one under each bunk mattress and then one on the inside roof of each bunk by putting it over the shepards pole in the middle and clamping it to the sides with black binder clips in the top corners. This still leaves the sides of the bunks themselves open, which can lead to a lot of heat gain or loss in extreme temps.
In the hot states, the blankets are probably better on the outside keeping the heat out, but here in the northeast with small kids, I need to keep heat in at night. It can get down in the 50's at night even in July, which can be very chilly for kids that dont know enough to keep covered up and having the furnace run all the time is annoying.