I would use standard ice cube trays, they are very cheap from discount stores.
Once the ice is frozen it may be removed from the trays, put in bags or boxes and another batch made.

Note that freezing water into ice requires the removal of considerable heat and the compressor in a domestic freezer will probably run continualy whilst freezing water.
Therefore only attempt to make ice in the freezer when line or generator power is available.
Make certain that the ice cubes are fully frozen, and the freezer is down to the normal temperature before shutting off the generator.