Just found an excellent source material for cassette bands -Maru slicone
swimcaps, for swimmers
So cutting sections of a spherical surface in effect
If 30 degree toe-in angle is ok with bulbous pulleys then , for the .4mm
thick rubber and 6mm wide cut strips diameter of 40mm (with toe-in angle of
30 degrees)
up to 70 mm diameter with zero degree toe-in angle (sphere diameter) range.
Silicone is about 50 percent more elongation for given load , area for area
, than normal rubber.
That is for the one in front of me, probably different sizes available from
babies upwards.