Dow 1" thick solid insulation. I painted mine and cut it to the appropriate size so that it fits snug , held in place by pressure. Keeps hot air out and cool air in.
If you need to add a little more security , so they don't fall out going 'bumpety-bump' while traveling, just add 3m sticky tabs or velcro tabs.
You can also use 2 sheets cut the same size and glued together. I cut a square 12" cutout and used clear plastic heat shrink storm window sheeting to fill the opening . It acts as a window to allow some light in, at the same time, keep the R value to insulate.