Twice yesterday I tried to start a crocheted loop to make a poncho out of. Both times I ended up with a mobius strip. So I decided to give it a rest. I'm glad I did because this morning my mind solved the problem. I'll just make a straight piece the right length, then crochet a few rows of double crochet to give it some stability and then join the ends together and start my rounds at that point. I believe that will work. Plus that will give me a little leeway if I want to add a button and clasp fastener down the road.
I got the idea from a poncho I saw in a book yesterday that was not only crocheted, but also beaded and embroidered with a cowrie shell fringe on the bottom. While I was doing research I came across this page of free poncho patterns which wasn't of use to me at the time but might be of use to one of my readers. http://www.crochetpatterncentral.com/directory/ponchos.php