As well as mechanical properties and lack of conductivity you need a means of creating the charge to be transferred. The following table gives the relative amount and the polarity of charge generated by various combinations of material.
There are may such lists available but this looks better than many, and includes some comments on 'incorrect" information available elsewhere.
A Santoprene rubber belt with a polyurethane roller looks like an interesting combination.
Alpha Lab Inc's The TriboElectric Series
Polyurethane foam on Teflon looks worth trying!
When researching materials, the final comment on the above referenced page is a warning about inaccurate data and may be worth noting:
A triboelectric series table has been circulating on the internet, and it contains various inaccuracies. Though attribution is rarely given, it appears to be mostly from a 1987 book. It lists air as the most positive of all materials, polyurethane as highly negative, and various metals being positive or negative, apparently based on their known chemical electron affinities, rather than on electrostatic experiments.
The page provides additional details to back their warning.
Related formulae: Electrostatic Formulas for Force, Voltage, Discharge Time etc. on Charged Samples or Surfaces