Hi, I completed a plug made from casting resin, and I have a small void. The plug is going to be used to pull canopies. Is it ok to fill in with some epoxy putty? Will the epoxy putty withstand the high temps? Some recommend car filler? 

It depends what the cast is made from.  Ideally you would likely be best to use the original resin to fill the void slightly proud of the surface, then wet sand smooth and repolish to get the gloss back.

Both the putty and body fillers all have a matt finish which may not be ideal for your project.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant