How to make XXL Watermelon Jell-O Shots without falling apart?
I'm totally retracting everything I said in comments. Doing this with just Jello (no unflavored gelatin) works just fine. The key is A LOT of Jello.

After reading your question I researched the question of "How much gelatin is in a box of Jello?" The only answer I was able to find was that a 4-serving box of Jello contains as much gelatin as a .25oz packet of unflavored gelatin powder (Knox). So, in order to test the ratio of the recipe with one 4-serving box of Jello, I added 80ml each of boiling water and vodka. 

    

Unbelievably, it worked. The firmness of the gel seems just right for the purpose.

This is cherry Jello, so the color is too burgundy. Unfortunately, I had to do it again to say for sure that the flavor is OK. I had no Jello, but my next door neighbor had some sugar-free. I so fully didn't expect it to work that I didn't care that it was sugar-free even though I knew full well that aspartame at that concentration was going to be inedible (and I was right - UGH).

So, I went to the store today and bought an 8-serving package of Jello. This time I used strawberry flavor with real sugar. In a bizarre coincidence, I saw something else in the store that I had never before seen in the state of Alaska: Mini Watermelons.

WOW, so I got one of those too.

Here we go again. I mixed the double sized box of Jello with 160ml of boiling water and whisked until dissolved (that took a couple of minutes), then I mixed in 160ml of vodka. As that cooled on the counter I scooped out most of the flesh from half of the watermelon.

After chilling overnight, it's all over now but the cutting.

 

The only reason I felt the need to lean the slices up against the custard cup in that shot is that I sliced them too thin. I was trying to make up for the fact that the watermelon is tiny. Cut these suckers a good inch thick (2.5 cm) and they'll stand just fine.

So, it works. Use 6 8-serving or 12 4-serving boxes of Jello to 4 cups (946ml) each of boiling water and vodka.

I had just enough jello/water/vodka mixture left after filling the watermelon to make a shot of that plus a shot using the same ratio of Jello/gelatin/water/vodka as in the original recipe.

 

This stuff is so boozy, that I think the additional flavor and sugar of the all Jello version actually tastes better than the original recipe.

I'm shocked, but you don't need unflavored gelatin to make this silly thing.

EDIT: 24 Hours later, I've got to add this. As I was carving the flesh out of the watermelon rind yesterday, I realized that I was being silly to be so thorough, especially at the ends. I really only needed the center slice for the experiment and a photograph. So just now I knocked off the Jello from one of the end pieces to eat some watermelon. That was really good! It had absorbed some of the vodka and flavoring, but the texture was just like fresh watermelon. So if you do this, do it a day in advance and leave some of the flesh of the watermelon. You'll thank me :).