Hi FruitGG
Yes, I think that is the right approach. I have discovered this Forum thread: New materials when the geometry is reinitialized - #6 by Andrea. Just add those 4 lines to your DetectorConstruction, and when you change something in the geometry, /run/reinitializeGeometry and all should be well.
I’m sorry this has taken us a little while to sort out. I hope it works for you and your membership.
John