Here is one idea, for example:

https://fstop138.berrange.com/2016/0...umen-printing/

There are some mentions of the spacing of the LED strips, but there does not seem to be a set rule.

Here is another box construction:

https://www.photrio.com/forum/thread...re-unit.176241

I still haven't gotten around to buying a soldering iron and learn how to do this, but some time in the future ...