I believe you don't need a box if you are connecting the signal grounds to an outlet ground.
If you are using a box by itself to dissipate EMI then make an antenna in a V shape inside the box with a simple banana plug. I fill my boxes with quartz crystals, Rochelle salts, and tourmaline. I use wooden boxes and also have purchased some cylindrical cardboard tubes for the passive devices.
So I use both methods and for the outlet I made a one wire plug for the ground only and a banana jack sticking out the back to make connections from the signal grounds to the plug.
I also use the Ground Master for power grounds onto the Puritan and separate ground rod. I may make a parallel run with another groundmaster for the signal grounds.