What material do the cubes need to be made of? is there a worry about the plastic of the cubes leeching into the hot wort after X number of uses?
Plastics are a very controversial issue, and it is unlikely that we'll have a clear consensus on longterm safety anytime soon. Some plastics are unambiguously unfit for food use (especially at high temperatures), while others are likely fine.
Generally speaking, the safest plastic container for no chill brewing would be a HDPE without plasticizers. If you can get this, make sure it is food safe and heat safe. The jerry cans used by most no-chillers fit these requirements.