What is a safe way to ground this light fixture?
Yes you can cut the hole but you need some protection for the wire coming through the hole I would find a large snap in gromet drill a hole to fit the gromet   I would move that 10-32 green screw to the point the wire comes through and tie your ground there but you can make a jumper in this case with a wire nut.
You need to protect the wires from getting cut  if you can’t easily find a large enough firewall gromet , snap in gromet , snap in bushing  you need some type of protection I like 3/4 snap in bushings the big box orange store has a bag of 3/4 acc snap ins for 2.23 a bag of 10.