Hi Tarc,
Some great advice from other posters.
Would the lettuce be the item to put in the middle and the cantaloupe in front?
I'd say to put the lettuce in front or the middle as it will bolt with the hot weather and you will need to remove it then.
Here's an a-frame trellis you can build for your vining veggies.
http://doityourself.com/vegetables/vegetabletrellis.htm
Newt
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.