Lemon-flavoured ice-blocks are easy.  Just dissolve glucose and citric acid in water.  (You could also use lemon juice, but my partner is sensitive to salycilates.)  Quantities are up to you, but as a rough guide, start with 1 cup of powdered glucose and 1 rounded teaspoon of citric acid and add about 3-4 cups of water, or more, depending on taste.

Unfrozen, this also makes a great lemon cordial without any artificial colourings etc.  Of course, you can add food dye if you want it to be yellow, otherwise the ice-blocks/cordial are colourless.  For cordial, use less water (eg 1 or 2 cups) to make a concentrate, then dilute when ready to serve.