If you have a dremel or other rotary cutting tool, you could use a small metal cutting disc and cut a a slot across the bolt, then use a wide edged tool to unscrew the bolt. Alternatively if you could find a large flat screwdriver with a head as wide as the widest points across the bolt hole you could jam it in and unsrew the bolt that way. Definitely both bodges, but the latter would be easy to try before investing in bolt extractors etc.
If you have a dremel or other rotary cutting tool, you could use a small metal cutting disc and cut a a slot across the bolt, then use a wide edged tool to unscrew the bolt. Alternatively if you could find a large flat screwdriver with a head as wide as the widest points across the bolt hole you could jam it in and unsrew the bolt that way. Definitely both bodges, but the latter would be easy to try before investing in bolt extractors etc.