I had a look at this before we bought our P38, and I'd thought of the 2nd hand reel bolted to the boot floor, but the problem with that, is the looping of the 3rd strap over the top inboard corner of the wider 'half' of the rear seatback, in a front end shunt, would impose too large a load on what is longditudinally unsupported - there's only the latch at the offside C post.
You could link the 2 halves together with a steel bar/latch support of some kind which would spread the load evenly between both latches.
Trawled all over the web and found (though can't remember where) a pic of someone's solution, a non-inertia adjustable strap fixed to the top centre of the tailgate 'goalpost', running forward over the seatback and down to the waist belt/catch. Clunky and fiddly but it did provide the requisite support.
I suppose with some ingenuity you could mount a reel on the goal post to make a more elegant and user friendly arrangement.
In the end I didn't bother as on the few occasions we need it the grandkids get the proper belts and their mum goes in the middle.
Certainly makes you drive more carefully . . .
Alastair
