I changed a couple settings from your illustration of my previous example breaking when adding a box on top.
(1st from developer mode)
and this one previously known but I didn’t use:
Now with your added box it seems to work again but it adds wall loops around the perimeter (probably as before also). I don’t like the weird layer layer behavior (NtoN, N-.1toN, N-.1toN+.1 are all different) and why increasing wall loops act this way at all is a mystery. The wall loop increase doesn’t need to be a big number, +1 or +2 seems fine. This method is the easiest to set up.
test - broken_2.3mf (55.8 KB)
Here is one with a modifier that changes the problem layer to solid and does not add an extra wall loop. It’s a little more fiddly adding and moving the modifier, but it’s probably a cleaner solution. The modifier is probably too thick, you could reduce it to one layer.
shells.3mf (55.6 KB)
Maybe try the last one with your real sign?