I installed harnesses in my -12 by wrapping plastic insulated 1/8 cable around the rear spar attach cluster, and used the left side over the right shoulder and visa-versa. I'm always annoyed when the harness catches around the back of your neck. The Y harnesses never seem to be comfortable for me, especially if the Y portion is too close to your back. This fixed it up nicely, and should comply with regs - and since you have a simple interior, should be an easy install. The rear harness could be attached by a cable to rear clusters, at a length to suit. You might also check CAR 8 about harnesses. The old Restricted category guys knew a lot about crash worthiness and how to properly install.