Can use rubber or cloth.
On my '56 I placed reproduction cloth strips as the insulation between the tank and the straps. For my truck I still have the old insulation on the straps but if I ever replaced it, I would get some universal insulation from one of the restoration places such as Dennis-Carpenter, NPD, Jeffs Bronco Graveyard, etc.
"Old Blue" - '56 Fairlane Town Sedan - 292-4V, Ford-O-Matic transmission, 3.22:1
'63 Belair 2dr sdn - 283-4V, Powerglide transmission, 4.56:1
'78 Cougar XR7 - 351-2V, FMX transmission, 2.75:1 9inch
"Bruno" - '82 F150 Flareside - 302-2V, C6 transmission, 2.75:1 9inch, 31x10.50-15 BFG KO2