OK, I’ve scaled the sensors to be evenly spread across 300mm in this sketch. I’d suggest that you print the pcb section of this sketch at 1 to 1 scale and try it where the final board will go and see if it will work with the sensors laid out like this or if we need to make changes (at this point with no routing changes are fairly easy). As well I added a 20 pin staight header (for header or more likely wires) and a dual row header for a ribbon cable connection if you choose. Again check that their position will let you get the wires out to where they need to go as again it is easy to move them now but harder later. As to the pins on the Arduino it is possible, but not necessarily easy to unsolder the header pins from the board and solder wires in their place with no headers. That is a stronger connection. I don’t know if anyone sells Megas without the headers soldered on (that would be the easy way to solve this problem.)
edit forgot to upload the sketch …
accordion-Sketch.fzz (67.5 KB)
Peter