Well, I think that the best geometric shape for a powder's container will depend on the geometry of your furnace and the extention of its homogeneous temperature zone. So, for a tubular furnace, which generally presents a central heating zone whose temperature homogeneity ranges about ten to fifteen centimeters, I recomend the boat container due to facility to introduce and remove it from the furnace... The most important is to assure the temperature homogeneity of your sample during the heat treatment.