Sluface Based Fluid Cavity Interface

[Nileo2005]

I am trying to model a cavity with two fluids, air and water. Due to the shear size of the model, CEL is right out of the question unfortunately, so I am going to have to result to surface based cavities, but am lacking a fluid interface to create a surface on. Anyone have any ideas of a material that wouldn't interfere with the reactions of the two fluids, i.e. act like a pressurizing diaphragm or displace too much fluid and invalidate results? I was thinking a hyperfoam material, but its only a surface thought.