Rectangular neutron tally surface above fusor

Hey,
Using GEOUNED I have exported a CAD-model of a fusor I built into OpenMC geometry. This fusor contains a sphere neutron source, and has two layers, one of stainless steel, and lead.
I want to create, for example, a 10x10 cm square, that I can place above the fusor, or beside the fusor, or other places I want to measure.
The square shouldnt contribute to the geometry itself, but be an ‘imaginary’ square, that just tells me how many neutrons has passed through it.
I’ve tried the .SurfaceFilter, but it takes an infinite plane as a parameter, and I need a finite surface/thin square.
CellFilter works, but for some reason it gives me a flux and current of 0, which doesnt make sense to me.
Attached is a screenshot of the simulation part of my code - C10 is a square cell placed just above the head of the fusor.