This was the prototype, I tweaked the setup later when I started making them for others
The HS1010A meter is used simply because it is the only meter I could readily find that was easy to mount to the end of the PVC pipe without a lot of extra hassle. They work good enough for these purposes in my experience, the biggest weakness is like most low end meters they will ready cool white lights higher then warm white but it is consistent.
The size of the pipe really doesn’t matter.
The tape is used for the final calibration fine tuning based on the calibration light.