Grounding a Ballast for wood enclosure when there is no metal?
Get a programmed-start ballast. 

Cheap ballasts also misuse ground to manipulate capacitive effects between the fluorescent tube and the metal enclosure.  Note huge assumption about fixture design.  

Programmed-start ballasts preheat the fluorescent lamp filaments (in the ends of the tubes) until conditions are ideal to strike the arc in the manner Tesla intended.  It results in a 0.5 to 3 second startup delay (depending on ambient temperature) but is very easy on the tube, and avoids the capacitive trick.  

Heat will be minimal on any modern ballast. In a fluorescent, the heat comes from the tubes.  

You may want to think about LED lighting and do an end-run around all these problems. 

You can't use NM cable as cordage.  If you want cordage, buy cordage.  They often sell it with a socket premolded onto the end, marketed as an "extension cord".  Make sure the writing on the side of the cord is legal for cordage, then SNIP!