Two sheets of 1/2" foam insulation board from Home Depot. I bought extra, but I have no way of returning the large sheets. These cost $20 each plus tax. I'm selling them for $10 each.
https://www.homedepot.com/p/RMAX-Pro-Select-R-Matte-Plus-3-0-5-in-x-48-in-x-8-ft-R-3-2-ISO-Rigid-Foam-Board-Insulation-637902/313501506#overlay