Here is a recipe I found in a cookbook called Kids in the Kitchen

Knox Blocks

Doesn't melt like jigglers.

4 env. unflavored gelatin
1 c. cold fruit juice
3 c. fruit juice, heated to boiling

Sprinkle gelatin into cold fruit juice. Let it sink.
Stir. Add hot juice, stirring slowly to prevent lumps.
Pour into 9 x 13 inch pan. Cool and cut in squares. Can spray pan with cooking spray.

Variation: Use 3 envelopes of unflavored gelatin and 1 small box Jell-O with 4 cups of boiling water instead of juices.