You'll need to do some fabricating for the harness mounts to the cage, installing them wrong will **** you up more than using a standard belt if not installed properly. Here's a more practical option that you can use with anu seat and a simple bar across the seat for mounting
3-Point Retractable Harness Belts - Harness Belts - Other - Corbeau Racing Seats
3-Point Retractable Harness Belts - Harness Belts - Other - Corbeau Racing Seats