I followed the jiggler recipe, but didn't realize that it called for 4-3oz. boxes - I had only bought 1. So I used 1/4 the amount of boiling water, and they turned out fine. The color of the watermelon Jello is a bit lighter than I imagined, so if you're going for the most authentic looking presentation, I'd suggest using another flavor like cherry. For some, I added brown sprinkles for the watermelon seeds, and the others are seedless. I had to come up with something to do with all the lime juice, so I made some limeade. Bob and I started thinking of other uses - like if you made these Jello snacks for a party, the lime juice could be used to make caipirinhas or mojitos. Then Bob came up with a brilliant idea for an adult party - make the watermelon Jello snacks into Jello shots by using tequila (or vodka, rum, etc.) and then press course/kosher salt on the outside! We haven't tried this, but I don't see why it wouldn't work.
I had 2 limes, so 4 halves, and had a little leftover from the 1 pkg. of Jello made the jiggler way, so that might give you a rough idea of the lime to Jello ratio for scaling up. Here's the recipe as it now stands:
Watermelon Jello Jiggler Snacks
Pink or red Jello gelatin (any flavor, watermelon, cherry, etc.)
Slice limes lengthwise, and juice them completely. Use kitchen sheers to remove the excess pulp/membranes from the insides of the limes. Invert the lime halves on a paper towel to drain off any moisture, if necessary. Turn halves over and arrange them on a plate so they are ready to be filled. Prepare Jello according to jiggler recipe on back of box. Transfer mixture to a glass measuring cup (or you can even prepare it in the measuring cup!) Gently pour the Jello into the lime halves. Some may spill onto the plate, but it can easily be cleaned later. If adding decoration, like sprinkles, add them now, or press them on after the Jello has set. Carefully transfer plate to refrigerator for at least 3-4 hours or until completely set. If desired, slice limes lengthwise again to create the look of a watermelon slice.
Source: You're Gonna Bake It After All