Before creating your plan, start by knowing exactly what the amperage capacity is on the outboard motor stator or alternator.
If your anticipated load with accessories plus a depeleted battery is an acceptable one for the charging system then you would need:
Case/lid or tray/bracket
Large gauge cables
Termination lugs sized for the cable gauge and the size of posts on both the batteries and switch (which may be different)
Dual battery switch
Loom, tie-downs, heatshrink
Plus, a crimp tool to properly make the connections or a soldering method