how to set up lumped port

I am simulating a spiral inductor on multilayer substrate. Lumped port (It is also called gap source port) are used. An airbox is enclosing the component. So how should I specify the ground for the lump port to connect to? I used Perfect E boundary condition for the air box and connected the port to the surface of the airbox. But I found the performance was larged affected by the size of the airbox. I read some docs saying I should use radiational or PML boundary condition for the air box. If so, how should I specify a ground for the lumped port? Draw a sheet and add Perfect E boundary condition to it? Thank you!

Do a ground ring around the spiral.

Thank you for the answer. I already tried it out. And I found the L & Q are quite sensitive to the ground size and position.