Dale has the same PVC light box that I have - same builder. It's based on a long ago thread here: https://budgetlightforum.com/t/-/6918
More info here: https://budgetlightforum.com/t/-/20730
The PVC light box is not perfect but I find it pretty darn accurate, or at least accurate enough. I use a calibration factor of 0.34, so take my lux reading and multiple by 0.34 and that gives me lumens. A light sphere is definitely a better design.