I recommend the use of ultra high molecular weight Polyethylene
(UHMW-PE) rather than high density (hdpe) low density(ldpe) ect.
Available .005 to 8" thick
very high dielectric strength 2300v/mil (900kv/cm) compared to the lower
mol. weight versions......
dielectric constant remains around 2.3
if anyone needs a source,i work for a company that produces uhmw.
Robert