Arduino RS485 Modbus Suggestions

Connecting three Arduinos, via Modbus over RS485.  See project description here:  https://forum.dronebotworkshop.com/project-help/10n-rocket-launch-control-system/#post-18738

RSOBox will be Master and the two (possibly more)  PadBoxes will be Slaves.  The Master will be polling the Slaves for a tri-state value (Open, Short, Ready) on each of ten igniter sub-circuits.  The Master will also be sending commands to the Slaves to switch relays and trigger individual rocket launches.  Fault conditions will travel across a separate wire in the bus, to ensure immediate handling, free of the MCU processing loads.

Suggestions for an add-on to the Arduinos for the RS485 cabling layer & for the Modbus address filtering?  Hoping not to burden the Arduino with Modbus messages that are meant for other nodes. 

Then for messages destined for the chosen node, to verify and pack/unpack bits for each rocket to single command bus packets.  Many of the Modbus libraries seem to be just master or slave, just "coils" or registers, or otherwise incomplete.  But why reinvent the wheel, surely you all have experience I can learn from!

Thanks!