Use hog panels for your pens, overlap them by 3 squares, put a tpost at the overlaps and one in the middle of each panel. Leave the middle post out or move it closer to one side on the panel that will be your gate.
^^^^^This is what I have done and it works very well. First picture is the first post to build the pen and it ends up being the hinge post for the gate panel
This photo shows how I tie the panels together for a fast install and I overlap every panel one square. I tie the first to the t-post then keep adding panels in a round pattern. I come back later after driving in all the T-post at each overlap and tie the over-lapping panels to each T-post in 3 places with 12 or 14 gauge galvanized tie wire.
This what each panel overlap looks like as I build a round pen.
This is the last T-post with the gate panel attached. You can hinge on the T-post or on a panel stay wire like I did on this one. Overlapping at least 2 or 3 spaces makes a more secure gate section.
Once you have all the panels tied together and have a round pen, you can adjust it to make fit the opening or as round as you like. Then drive in at least one T-post at each overlap. Then if needed one more in-between each T-post. If you use 10 panels then you will need 10 or 20 T-post.