Test instruments with metal chassis faces the same problem. The chassis should never be floating with respect to the circuit because static charge could build up and especially so in your application. If no ground plane is used, then a single point ground is usually the best option and if possible, should be at the signal ingress/egress. With a ground plane, every standoff might be connected to the chassis.
In applications where a direct low impedance connection is not feasible, some provision should still be made to bleed off static charge like an inductor or resistor between the chassis and the single point ground. Often in this case a bypass capacitor is also used to make an RF connection, or a breakover (?) capacitor which acts as a spark gap.