Check the original build of @DavidKingsman first and read all intructions how the build is done.

This remix just addresses changes on the MiddleFrame, Front- and BackCover in order to support 21 characters.

I took the Covers And Frame pieces and changed them to my needs. The FrontCover and MiddleFrame pieces are assembled in the same manner as the original. The Parts B+C+D+X of the BackCover will use some nuts of the UnitFrames as depicted in the picture. So just enter the M3-nut into the UnitFrame, but do not use it to fasten the UnitFrame to the MiddleFrame. Use M3x20mm bolts to fasten the BackCover(B/C/D/X) towards the UnitFrame nut.

If you go for a build as big as this, think about adding additional power supply lines. With 21 characters flipping at the same time, this build draws ~55Watts from the wall. This exceeds the JST-XH standard allowed 3A. One supply line for each 10 characters should be sufficient.

You need to edit the addresses of each Nano beyond the 16th caracters by yourself (and you can do it with the rest (1-16) as well. No need for DIP switches.)

Use extensions between parts B-C: A-B-X-C-D-E
Any amount of extensions should be possible.

I removed the fishtail interconnection of the BackCover for easier access to the Nanos.

I added little support-cubes into the FrontCover pieces on which the UnitFrame can rest.

Maybe forget about the wall-mount holes - 21 characters weigh about >6kg.

Check out the blender file to in order to know what is going on in this build. Beware: I am no blender expert, but it turned out well for myself - feel free to improve the blender file.