A person would need to know much more about your installation to recommend a relay and provide a diagram.
What is the control voltage? Most likely 120 VAC or 240 VAC depending on what part of the world you're in.
What current and voltage will the relay be switching? Will you be switching power to other devices?
The E-stop input to your BOB probably needs a contact closure to ground to allow the machine to run but I don't know the details of your setup.
Where will you mount the relay - How much room is available?
A relay similar to this should work unless you need more than 2 contacts.
https://www.ebay.com/itm/DC12V-AC110V-Coil-Power-Relay-DPDT-LY2NJ-HH62P-L-JQX-13F-W-PTF08A-Socket-Base/401477573638?hash=item5d79ed9c06:m:mtNxrUqG3zT3AHABmKrVn1AThe primary purpose of an E-stop is to protect people from injury. Also it prevents damage to equipment.
The E-stop input to MACH3 is a software function thus a computer failure could leave something running. It's unlikely a stepper would continue to receive pulses but the speed input to a DC controlled motor may remain on.
If you are using a motion controller it may have circuits that stop motion independently. Also if the 'Charge Pump' circuit in a controller is used it will stop motion if it fails to receive a steady stream of pulses from MACH software.
Still there is nothing as certain as interrupting the power to the motors with a mechanical contact. You need to decide how much risk there is and how much you are willing to do to limit it.
You may be able to add additional E-stop switches in series. There is a note saying 'SEE LINE 14A' that should be checked first. The E-stop may do more than enabling the motor starter.
Graham Waterworth asked about the E-stop switch. Adding a contact or replacing it with one that has more contacts is good solution if you only need to add one more circuit and don't foresee adding more E-stop switches.