In Animal Farm, Clover is introduced in Chapter One as a "stout motherly mare approaching middle life." As such, you could use a shoe-horse to represent Clover physically, or even a piece of clover, while objects like baby clothes or a bottle symbolise her status as a mother. The narrator also informs us that Clover has given birth to four foals so the number four would make a good symbol.
Alongside Boxer, Clover is one of the most dedicated animals on the farm. After the revolution, for example, Clover avidly attends all the meetings and absorbs all of the information taught to her by the pigs. She is also extremely hardworking, as we learn in Chapter Three when she harnesses herself to the cutter and the horse-rake. A harness or leash, then, could be used to symbolise her hard work and commitment to the efficient running of the farm.
Clover also maintains and defends the principles of Animalism when needed. In Chapter Five, for example, she confronts Mollie about allowing a farmer to stroke her nose and she finds ribbons hidden in Mollie's hay. For your book bag, draw a coloured ribbon with a red cross through it to symbolise Clover's strong principles. Similarly, a heart could be used to represent her love for her fellow animals.
As Napoleon's domination increases, however, Clover is saddened and deeply moved by the failure of the revolution:
As Clover looked down the hillside, her eyes filled with tears. If she could have spoken her thoughts, it would have been to say that this was not what they had aimed at when they had set themselves years ago to work for the overthrow of the human race.
Tears not only symbolise her sadness but a whip or collar could also symbolise her domination by the pigs and the failure of the revolution, more generally.
Finally, use glue to symbolise the loss of Clover's greatest friend, Boxer, when he is sold to the horse slaughterer by Napoleon in Chapter Nine.