Not seen that before, but just thought I'd mention another popular method I use occasionally, mainly because I don't have a really big softbox and would only use it very occasionally. But I do have several smaller rectangular softboxes, and a few heads, so I stack them one on top of another on the same stand, or side by side on two stands, and it works really well.
It occurs to me that, for example, four Lencarta 70x100 Profold softboxes at £120 each would cost roughly similar money, and they go up/down in the same easy manner as the 6x4ft one linked. That would give several options, such as a pair of giant strips, or one big box 4.5 x 6.5ft. And four times the potential power too of course. Brightness might be more even than one huge box, and you can grid them as well
Edit: Bessel do a big 120x60cm at £60, or maybe four 60x90cm at £50 each that would make one 4x6ft for £200.