An apparatus comprising a first structure, a second structure, a first layer, and a second layer. The first structure has a first plurality of channels extending towards a number of edges of the first structure. The second structure has a second plurality of channels extending towards a number of edges of the second structure. A first side of the first structure is connected to a first side of the second structure. The first plurality of channels and the second plurality of channels have an orientation with respect to each other. The first layer is connected to a second side of the first structure. The second layer is connected to a second side of the second structure.