Choose the bricks you want to use for your ice box. Bricks come in all shapes and sizes, although the most important thing is that they are small. Ideally, you should avoid bricks with a lot of holes since these air gaps indicate that these are less hot bricks and overall have less heat in your cooler. The entire bricks without holes stay hotter for longer, so they're ideal if you have them. If you can't obtain any bricks, use stones or huge boulders instead. Just be wary that stones may shatter when heated.