Hi!
I had a similar situation and great success with "diamond steel mesh" from Lowe's.
It comes in sections 8ft long by 27 inches wide for about $8 per section. It's used in masonry... probably concrete reinforcement. It's sturdy (about 1/8 thick), and a small mesh. Very strong, but won't rust. Top and bottom edges are smooth along the 8 ft. Cuts easily with snips.
I built a circular exercise area for our 2 ducks out of this and steel U-posts, using zip-ties to hold the mesh in place. 2 sheets high makes the pen about 45 inches tall (with overlap).
The gate was the best part, though. I found that if I rip through 1 side of a 3/4 inch pvc pipe, I can slide it over the edges of the mesh. The pvc is cheap, easy to work with and adds a lot of rigid strength to the gate. Also eliminates any sharp edges.
Don't know exactly what kind of pen you need, but I can testify that this is the strongest and easiest stuff to work with that I've found.
Let me know how it works out for you!