Nice setup.
You can get panel switches in just about any configuration. I would use a 2 position, maintained operator. Add as many contact blocks as needed.
https://www.automationdirect.com/adc...inated/GCX1300
How you configure everything is dependant on your hardware and software. Depends on how many unused I/O points your controller hardware has. I don't think you need ModBus to do this.