Back panel should be slightly smaller to have tolerance between the extrusions

[elcojacobs]

The back panel width is exactly the same width as the x extrusions (54cm on the 400).
This leaves no room for error when the panel is made and there is no good reason to have it this tight.

The extrusions should be tightly pressed against each other, not against the panel.
If the panel would be 538mm, it creates fewer problems during assembly.

We discussed this on discord and Runeeh provided this photo:

The height is also best reduced by 1-2 mm.
Mistuma provided this photo of the top extrusions being pushed up by the panel: