We used 7/16" OSB on the walls and screwed them to the old window frames using 1 1/4" sheet metal screws. Then we drilled holes in the wood and pumped Great Stuff big gap filler into the walls. It was more expensive then sheet insulation but it did a really nice job filling every nook and cranny and will hopefully be quieter. It also pushed against the sheet metal and made a nice solid wall overall. Notice the Rustoleum aluminum colored paint that we used on the ceiling. This did a nice job of covering up the light mint green standard bus paint.
