I don’t use clips. I use the method shown in this video.
https://www.youtube.com/watch?v=6MVQYRo1WhQ If you don’t like to click on links it is called -Watermelon Grafting Demo.avi - YouTube.
I made a few changes to his method.
– I do a longer cut into both, not deeper but longer down each stem. I usually do it till I reach the top of the razor blade. I don’t cut into either stem more than half way.
– I use teflon plumbers tape, I found that the tinfoil can cut into the tender stems as they grow out a bit ( this may be because I tend to leave them together for longer than suggested).
– I cut off the pumpkin top at around 5 days, and I leave both roots attached until day 10 or even 14, then I cut off the watermelon rootstock. I have cut them off earlier but I found I had better success if I left them on longer.
You can see the tinfoil cutting into the stems in the above picture.
In this one I used parafilm, but later I started using teflon tape and like it much better
Another picture of the grafted ones in the back and the controls, or non grafted, in the front. I found the size difference to be significant.