The good news is . . . . . you won't need to practice on 20 bananas! But you might plan on using more than what you want to make because there is a trick to getting it right. I will try and walk you through my experiments so you won't make the same mistakes that I did.
I used store bought flavored jell-0 for the jello filled banana because it worked better.
Fruit juice (I am not sure about pineapple juice though)
Knox unflavored gelatin or Agar Agar.
Greenish firm Bananas
Lemon juice approximately 2 Tablespoons, used to help prevent browning on the bananas
Sweetener if desired
Funnel, ladle or spoon
Tiny heart shaped cookie cutter
Tooth pick is helpful
A bowl or something to prop the banana up as well as the banana end as shown in the pictures.
I made some banana jello that was made using apple juice, Knox unflavored gelatin and the banana. I blended the juice together with the banana and followed the recipe. It tasted very good. The color was off unfortunately, so I did not use it. I thought it would be awesome to make a banana jello filled banana! It tasted just like a banana only it had a different texture!