Suggestion on building a bay pen

I like the belt idea as well, I have thought about doing that with the larger of my two pens with old carpet. I could still use the smaller portion for letting pups bay from the outside. I have two round pens that touch making it shaped like an 8. I have a gate where they touch for sorting hogs.
I used the 5' tall 4"x4" panels and some T-posts I already and I still have around $300 in it. Yeah I know there are cheaper ways to go but it was my only option at the time I was needing one. I built a woods pen that is about 3 acres at my grandparents house and dug a pond in it and the whole nine. I just used field fence and it worked great. If I had 10 brushy acres that I wasn't using for much I would clean out a fence row, fence a few acres, build a round pen in the corner of the pen and feed water my hogs in that. I would put a trap front on the round pen and then if I wanted to turn them into the woods pen I could do that and then catch them back up in the round pend. Not cheap but it wouldn't leave you wanting. I would look to spend 1200-1500 in fencing material minimum to do this.
Matt