Just use a urethane selant like Sikaflex to repair it. put the battery on its side with the cut to the top(empty acid into glass or plastic container if its not sealed. Rough up the area around the cut with course sand paper and urethane another patch of plastic over the top of it. Give it 12 hours to cure before righting the battery(and refilling it if required).

A tube of sikaflex will cost $15 to $20(caulking gun type).

Its worked for me before and can look neat.