this recently rekindled thread from 2012 describes more of what Cereal Killer referred to.3 right angle bends in 4”/100mm PVC that slides together , no need to glue.Parts are available worldwide , locally and cheap.Paint the insides matt white and fit a card with light meter hole in the middle at one end.
Djozz has shown that sub $20 lux meters do as good a job as $500 lux meters so it would only need a decision on what model…groupbuy!
Non modders can join in and it can measure torches up to 100mm lens diameter.Seems easier to me.