Those Bandmaster and Bassman cabs were filled with fiberglass. The concept is called an infinite baffle where there is no output from the rear side of the speaker. Like the cabinet was infinitely large.
Later Kustom cabs were ported so they were only lined with the fiberglass batting.
You can use the type of fiberglass that is used for wall insulation, but it is rarely available in one inch thick unfaced rolls.