Susan
I recently asked something similar on another group and what I decided to try is PVC pipe (plumbing) and fittings you can get this fairly inexpensive at a home improvement store.

I have a spot in a community garden and I need on a little over 7' wide so I am doing 3-6' tall pieces and then 2 3'8" pieces connected by a T fitting in the middle and an elbow fitting (90) for the corners. I will use plumbers glue to join them but ony because we already have it here

To anchor it in the ground I have 2 18" pieces of 1 1/2" PVC that I will pound half way into the ground and then in the middle of that I will have 2 scrap pieces of rebar for extra reinforement setting the frame over the rebar and into the bigger pipe

I am going to use nylon twine for my plants to grow up and also run some horizontal also for more support. I have read of people using various kinds of netting stretch over the frame before you put it together. I suppose it just depends on whats growing on them

I also seen a neat idea to reuse 6' fence panels (chain link) for trellising veggies.

Good luck
Amy in MS