Easiest would be loam bricks, those pressed mud, unfired bricks also known as adobe or raw bricks sometimes. Although in the higher region of the bell, namely everything above the riser's end, should be shielded by one or the other kind of heat resistant material. Like firebrick, superwool blanket or the like.
Second easiest would be just fired brick with mud mortar. Or build it out of firebrick on edge with normal brick or adobe on the outside. It all boils down what materials are available and affordable to you. Even hollow earthenware bricks filled with mortar or mud could serve as the main building block.