Insulation only work if it is complete with no air gaps or voids.
I made pillows for the sides of the refig. by placing some fiberglass insulation a bit oversized to the gap and then stapling 6 mil plastic sheet across it. This allowed me to slide the refig. in and the insulation made good contact with the sides. At the top I made a sheet metal lip that contacted the refig as it was slid in. This was not a good insulation system but conveyed the hot air towards the vent. To the rear I made a sheet metal false wall to produce the 1 in. gap that the manufactures require for proper installation.
The picture should show all three ideas
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles