Why?
Why do you think you need an inertia reel? What do you expect to gain from the added cost and effort? I have been completely satisfied with the fixed belt system from VAN's, it is comfortable and I have no issue reaching anything on the instrument panel, including the heater cable on the far passenger side. Have you flown in an RV to check out the standard belt system, if not I would recommend that you do so. My humble recommendation is to forget the inertia reel, keep it simple, keep it light, finish sooner and get in the air. You will not regret it. Good luck.