Finally decided on a hot-rod strip as the long softbox to get - there are a few positive reviews out there like this and this (which is really a video review), and no negative ones I found.
As regards general use and assembly these reviews are good, but I soon noticed that if the box is close to the subject it has a tendency to give a 'hot middle'. You can also see this in the photo's of Sarah in the first review. Indeed the reviewer says "At close range the illumination starts to fall off in a full length shot. So here the the feet are underexposed." In fact imho the central part is tending to overexposed - in both the full length and the 3/4 shot. A quick photo of the box itself shows why this isn't really surprising.There is a big fall off from the centre to the ends of the box. In fact looking at the histogram shows there is just over 2 stops drop off from the centre to the ends.
So I wondered if it would be practical to improve this situation. Here is the result of an afternoons messing about with silver foil and fabric stiffener.
A big improvement, confirmed by the histogram below. Now we have about 0.8 stop variation in output along the length of the box. And yes - this does make for more even lighting when the flash is close to the subject.
I'll publish info on how to do this tomorrow.
I've also added some piccies of the illumination from the original and modded versions here