Psssst - I mentioned it!!
Nice review. You certainly invested heavily in it - I hope it works for you. I bought a single ring and adapter for use on my ND400 since it's the one filter I find myself removing as a part of shooting, figuring if I liked it I'd buy more. Great idea, but even with a nominally shallow filter they essentially double (or better) the depth of the filter. I'm interested in what you find, but for me I found that the 16-35mm vignetted thru 24mm, and the 24-120mm vignetted through 38mm. So, technically I'm covered from 24-120mm across the two lenses, but wide open at 16mm I've got some significant vignetting to deal with...
24-120mm at 24mm
16-35mm at 16mm
To truly make this work I believe it requires you to buy a filter 1-2 sized bigger than you need, and they should incorporate the step-up into the filter adapter. Of course, this nullifies the ability to take the hood on and off with the filter on, and would likely require folks like us who have filters to go out and buy replacements, which no one wants to do. My brother and I spent a long time talking to the guy there and there isn't a perfect way to make this happen that also allows for the lens hood, but I would rather know that I cannot use/remove the hood when using this than to lose this range of focal length.
With that said, I decided to keep what I bought because it
will come in handy when I'm using the ND400 on a dim day over top of rocks or moving water.
I know YouTube comments are a PITA, but if you turn on moderation then you can at least filter the BS and I believe folks who find your video to add experiences like mine to help with their decision making process.