How to prevent ice from melting ?

I am preparing some ice planks for my Tae Kwon Do demonstration . My friends and i will about to kick the ice planks tomorrow. We have an ice box to keep all the ice planks we made but we need to keep it from melting as the demonstration starts after 3 hours we take the ice out from the freezer .How to prevent the ice from melting or melting too fast ??? please help me