You need a piece of ... something. Which fits across the film rails, which are the shiny things above and below the opening when you see the camera from behind with the back open. You can use a piece of film, even - I've done that. Surely everybody has a piece of film that is sadly grey and featureless?
Put your GG substitute across the film rails, open the lens (on "B"), set lens to infinity. Pointcamera towards something very distant. Does it look sharp? Anyway, twiddle the focus a little. Is there a sharper setting? If so, your infinity is off.
I did one where I cleaned the front group of a lens, and had entered it in the wrong place on the helicoid...
I changed the lens on another camera, and needed to take off about 0.5mm from the lens mount to get a good infinity focus...
Thickness is not important at all, but the "focus surface" has to be towards the lens!