This is the card I always use for those applications & more:
https://www.jaycar.com.au/pressboard...erial/p/HG9985
It is around 1mm thick and for cardboard it has an amazing dielectric strength. It also looks very good varnished.Have a look at the battery case made out of it on page 12-13 of this article:
http://worldphaco.com/uploads/USING_...CILLOSCOPE.pdf
PS: the original card Jaycar supplied was a redish color, like that in the article, of late it has changed to a pale brownish color but it is electrically identical.