Window unit airconditioner panel insulation

I have a double paned windows with a window airconditioning unit installed. The unit was profesionally installed. However, the side panels connecting the frame to the conditioning unit are a flimsy plastic that allow heat and sound to infiltrate. What is the best way to insulate the panels from sound and heat loss.
Thanks in Advance Endo
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Put an inch or two of foam insulation against them. Can be done on the outside if you can reach them.
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