Since these are so hard to find, I’m going to record all findings.
According to a 1961 document called Plastic-Film-Dielectric Capacitors by C. B. Charlton, high density polyethylene is 2.2, and low density polyethylene is 2.25. Polystyrene is 2.55. Polycarbonate is 3.0. PET is 3.1. PTFE is 2.1.
Polyesters have a constant of 3.1 approximately, according to a few sources. This website sells polyester enamelled wires. The 200 degree wire might have a coat of polyamide/imide too, but I think it does not change the dielectric constant much. Using the formulas and excel spreadsheet earlier, a thin wire (36SWG) will therefore provide around 45 ohms characteristic impedance (not much change due to number of twists), taking into account the ferrite.This is using a guesstimate of the insulation thickness, using this table. Also, I assumed polyester was a hard insulator.
To get higher than 45 ohms, a different dielectric than polyester is needed (i.e. a lower dielectric constant), or a very thick insulation.