[wlesieutre]

You need to control the direction of the freezing. As water freezes it will push the dissolved air out, and if it's freezing inward from the outside faces (like a normal ice cube tray) those bubbles end up pushed inside your ice block.

Using a cooler means the sides and bottom are insulated from the cold and won't start freezing as quickly. As it freezes top down there won't be any bubbles stuck there.

Demo on clear ice using a cooler from Cocktail Chemistry on YouTube: https://www.youtube.com/watch?v=bUHcCHbgX_o

[chongli]

Another trick is to drill holes in the bottom of the wells of an ice cube tray and rest the tray on a frame above the bottom of a basin. You fill the water up so that it's level with the top of the tray. When the water freezes, the air gets forced down through the holes in the bottom of the tray. Then you can chip the tray out and remove your clear ice cubes.