Here's a trick: make a sling out of something that won't stay wet, like a nylon stocking or onion sack; tie the sling to the trellis. The sling will bear the weight of the melon. You don't need to do this until the watermelon is a bit larger.... maybe the size of an orange. This works for anything that will get large. BUT... Is your trellis strong enough to carry the weight?

Got access to any old brassieres? A few DubD's outta handle it clipped to a support. No pun intended. And there are no mistakes in gardening. Only lessons to be learned. Take pictures please.

Fields (i am giving you a nickname:) we can always count on you to have fun comments.
also- do you say you are "inpatient' on purpose or are the m and n transposed on your ipad? or are you waiting to become an outpatient? ha! min

I had a container watermelon once and trained the vines up a trellis as well. I used a net bag that some fruit came in to keep the fruit held up. Worked great.