What’s the best or quickest way to fix a chip in an epoxy board??

Best: boardlady's way. But that is intensive and expensive.

Cheapest: you say chip? If it is not thru the layer of (brownish coloured probably) foam that immediately below the outer layer of glass, you don't need epoxy. Epoxy is only needed as normal fibreglass resin based fillers and the resin itself eat styrofoam. Therefore if the styro core is not exposed you can use a cheapo filler like Plastibond (Bunnings etc, about $10).
Filler 'er up, let cure for 2hrs in the sun them sand off.
Make sure u go down to 600grade wet n dry (used wet) and it will be smooth as. Paint with cheapo spraypaint if you wish.
Lots of unpainted repairs look hardcore if you're a wavesailor.....