I knew you were going to say that!
I watched your gif very closely, but I’m missing something since I couldn’t get it to work. While I’m thinking about it, I’ll try again and indicate what I see.
Probably the GIF that Box made?
The files you uploaded have groups. Groups don’t work with the “Dave Method”. It requires the use of components.
Scale up by a factor of 1000 and set units to Meters while you are modeling. Also, don’t use the native Solid Tools. They convert the components they modify into groups. Use Eneroth Solid Tools or Bool Tools 2 instead.
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Yeah, I made it a component.
At least I thought I did, but I just tried again and it worked fine. So, I’ll always make a component, create a clone, scale, edit, then delete the clone.
BTW, how did you get that measurements toolbar?
I have “Mearuements” checked in “View Toolbars”, but I don’t see those options when I click on tape measure.
Yeah. Make sure you exit edit mode before deleting the giant copy.
Are you talking about @Box’s GIF? He’s got a custom toolbar that he uses for these things.
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Technically speaking you can use groups but they must be wrapped up in a component. 
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Thought you might be interested (or not). The first pass print came out really well:
Only issue that surfaced was a slight shrinking of the LED holes, so the 5 mm LEDs don’t quite fit (laser cutting adds clearance due to kerf, but 3d printing typically shrinks holes slightly).
Good thing the front piece was still a solid–it was easy to create a slightly larger diameter cylinder, position, and subtract to cut out larger holes to provide sufficient clearance with PETG shrinkage.
Thanks for all your help!