Yes, we have. Extensively. It turns out that the shape, size, and angles of orientation all matter. A lot. Surprisingly, these additional aspects make the area of contact somewhat less important in the overall design. Certain dimensions have more to do with rate of change in the degree of criticality versus time, due to the specific angles of enclosure as seen from different points in the chamber. It is more of an IR effect than a surface of contact effect. This sort of esoterica can quickly get rather complicated to explain.