Slap your clay onto a smooth surface to begin removing any air bubbles. If you have air bubbles, your clay box can cave in or break during the firing process. Cut the clay in half lengthwise with a knife. Pick up both pieces and slap one down hard onto the work surface. Slam the other piece down on top. Cut it half again.
Place two yardsticks parallel on your work surface approximately 15 inches apart. Place half your clay in the center. Roll it with a rolling pin between the two yardsticks to roll your clay at an exact thickness. Flip the clay over and roll it again until it is an even thickness all over.
Cut a rectangular bottom box piece from the clay that is 4 inches by 6 inches. Use a ruler and your knife. Cut two side box pieces that are 3 inches by 6 inches. Roll out the other half of clay to the same thickness as the first half. Cut out two side box pieces that are 3 inches by 4 inches.
Dry out your clay pieces for approximately one hour at room temperature. You want them to become leather-hard. Mix together roughly one-quarter cup of clay with equal parts water in a cup to form clay slip, which is essentially wet clay with a porridge consistency, for the box's mortar.
Lay the 4-inch-by-6-inch clay bottom piece down in front of you. Brush slip onto a long side edge using a craft brush. Press a 3-inch-by-6-inch wall into place along that edge of the 4-inch-by-6-inch bottom piece to create a side wall. The 6-inch sides meet up. Brush slip on the edge of the opposite side of the 4-inch-by-6-inch bottom. Press the other 3-inch-by-6-inch wall into place.
Brush slip on the edge of one of the short sides of the 4-inch-by-6-inch bottom piece. Brush slip onto the edges of the two other walls from the 3-inch-by-6-inch pieces. Press one of the 3-inch-by-4-inch wall pieces into place. The 4-inch side is at the bottom. Repeat this step with the other 3-by-4-inch piece at the opposite side.
Roll out four 3-inch snake-like coils that are roughly one-eighth-inch thick. Press each of these into the four corner seams inside the box. Smooth them gently into the corners with your fingers. The coils strengthen the wall preventing them from caving in.
Crumble half a sheet of newspaper and place it inside your box to keep the walls from caving in. Cover the box with the other half of newspaper to let it to dry slowly over a week. Once it dries completely, fire it in the kiln with the crumbled newspaper inside the box to keep the wall from caving in.