I removed the old style large TV box over my dash and built a cabinet. I wanted to insulate that area behind there all the way over to both side walls. It had no insulation and was just empty space. I did not like the idea of fiberglass dust particles raining down on me as I drove.
I like the 100% polyester "no-Itch" insulation that used to be available in Lowes. It is no longer sold there and I think Home Depot only sold an entire pallet of it. As an alternative for small jobs, I use pillows.
You have to find the hypoallergenic, 100% polyester fill ones that just put the batting in without chopping it up. It is fire proof and mold resistant. I found the Sertapedic brand at Wal-Mart had this. I think the job took about 4 pillows.
I also just used it to insulate a large bare area over my fridge as accessed from the outside vent. I am ordering the ARP fans and wanted a limited top cavity. I formed a wall in front of the insulation with sheet aluminum.
I always remove the pillow cover as it is flammable.