Setting rear wheel alignment

[jps]

I have seen various posts and resources about setting the wheel alignment, and understand the 'string box' process, which I plan to use.

However- the first starting point - defining the centre line of the car, and then consistently working back against it, is what I am struggling with a little.

One suggestion seems to be to attach a length of box to the car (wider than the track) - probably behind the rear axle, at 90deg to the car centre - to give a definitive reference point for the 'string box'.

Has anyone done this in practice and can share how?

I can see no end of potential for imprecision if I simply fit a couple of rivnuts to the chassis, then bolt up a length of box using say an 8mm hole with m8 bolts (given the amount of 'slop' in a standard hole!)