Ok this is going to sound weird, but what I use for the diffuser is diffuser paper. It came free with my DSLR speed flash back in 2017. When I first starting metering using the app (Photone but at the time it was called Corona - oops) I remembered the flash had a whole collection of diffuser paper, most of it colored for artistic photos, but it had exactly 1 plain piece of diffuser paper in the kit.
Sorry you can kind of tell it's been used somewhat regularly for almost 4 years now. I recently split the cost of an apogee with a friend so I got to test the Apogee vs the iPhone 12, on average I got between 3-5% difference between the two.
Literally the lowest effort diffuser effort I could come up with and I used it for 4 years. Just a tiny piece of paper the sits on top of my phone camera when I use Photone.
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