An assembly unit includes side plate having first coupling member on first side, second coupling member on second side, and third coupling member on each of third and fourth sides, top plate including fourth coupling member and top stud, and bottom plate including fifth coupling member and bottom stud. The first coupling member is engaged with the second coupling member of neighboring side plate. The fourth coupling member is engaged with the third coupling member. The fifth coupling member is engaged with the third coupling member. The first coupling member includes protrusion. The second coupling member includes groove. The protrusion is fitted into the groove of the neighboring side plate. The third coupling member includes thread. Each of the fourth and fifth coupling members includes thread. The fourth and fifth coupling members are engaged with the third coupling member in a screwed manner using the threads.