Just a super quick review here.
If you like the occasional spirited drive, on track or b-roads etc I'd highly recommend picking one of these up (around £45-50).
They basically lock the seat belt at the hips, so your fat butt won't slide around any more, takes a few moments to install or move to another car.
I believe it's been re-designed a couple of times, so it won't fall apart (which did happen to a mates one years ago), it does exactly what it says