I don't know if it would make a difference. It might make it easier for you to pick if it's against the wall. There's not going to be any sunlight on the other side of the trellis if you put it in the middle anyway, so I think you should just put it against the wall. Of course, I've never grown melons on a trellis against a wall, so I'm not sure how sound my advice is :)

I grow those on a trellis, no problem. Make sure you plant something(s) nearby to attract pollinators. I put mine against the wall, but in that zone you might cook them if full sun all day.

Dan

Dan~ You make some great points. I think I'll put the trellis against the wall to allow some space at the front edges of the box for flowers. I'm prepared to hand pollinate if needed too...

Regarding "cooking" the melons in the sun, it does get warm there, especially with the stucco wall, and concrete path in front of the box. Would starting earlier than normal work? If not, I have a list of other melons I'm thinking of... So hard to decide with such a small space!!! Any thoughts?

Green Machine
Sleeping Beauty
bush Sugar Baby Watermelon

You can try to start earlier but the maturity period will still take you into heat, esp if El Niño carries on through the summer. You can shade the wall to lessen the reflected/absorbed heat.

Dan