All I need is to modify the library and get a long “corner-to-corner” connector which is another story.
Should be pretty easy to find a 16-pin IDC cable: https://www.amazon.ca/s/ref=nb_sb_noss/134-5881422-4725914?url=search-alias%3Daps&field-keywords=idc+16+pin
If you want to make your own custom length, the cable and connectors aren't that expensive, and though you could buy a special tool to compress the connector, a vice works just fine.
Also consider using a short cable and just adding the "folded" second panel upside down. SmartMatrix Library supports Z-shaped and C-shaped chaining (referring to the shape of the path the data travels through the panels and cable for each row).