I wrap the shoulder belt around the Boxster roll bar. The geometry works for me because I am quite tall. Sometimes wrapping to the rollbar causes too much of a downward angle on the shoulder straps. See here for mounting guidelines:
https://www.schrothracing.com/docs/Schroth_Installation_Checklist.pdf
I recommend a 6 pt harness, and if your seat and rails didn't come with a sub bar, check brey Krause and then the various vendors and the classifieds to pick those up.
I highly recommend a head and neck restraint device when using a harness.
Racing seats are a great mod, it really helps when you don't have to brace with your leg and the steering wheel, and I get a lot of useful feedback through my shoulders via the wings of the seat.
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