2x12 halfback cabinets

[hogridr]

I have an old 2x12 halfback cabinet, I recovered it since the tolex was shot, but other than that it is in great shape. My question is about the bottom half. I was told that there should be some insulation in the bottom ported section. Does anyone know what sort of "insulation", foam, ??? A picture of the proper material would be great. Thanks.