The problem is I bought the components before realizing the difference between SMD3058 LED strips, and the much more powerful SMD5050 LED strips. When I noticed my mistake I decided to carry on because its possible to get something like three times as many 3058 LEDs in a given area, so I hoped quantity might cancel out individual LED brightness. That didn't happen.
This is where I was up to a few days ago and shows the nearly finished panel and outstanding materials

And this is the finished panel

The metal strip at the bottom spreads the mounting point load to the outside of the box. I felt the box would twist and break without this.
Switch and DC power socket. I used to do this stuff for a living, but it sure comes hard with my much older eyes.

And the proof of the pudding... :-( This shows the DIY using compared with one of the Photo & Video panels. Both exactly one metre from white foam board. WD set to daylight. Exposure measured with my Sekonic L-758DR and files completely as taken with zero adjustment.
The most noticable difference is the blue cast with the DIY unit. The sampled RGB values show the extent of the error. The other noticable difference is the 1.66 difference in panel brightness. Neither light comes remotely close to my big Bowens flash units (four or five stops) but they are still useful.

I have most of what I need to make another panel but will need the 5050 LED strips (two off at $50). I also started with a PSU that will power several panels, so I will definitely carry on.