I did 30" lengths of 1/4" once and it bows like crazy. Without a brace on the top I was getting almost a 1" bow at the top. Even with an aluminium angle top I still had a wicked bow in the middle of the pane. I would go a little bit thicker if you can afford it, maybe 3/8", 1/2" if you can spring for the cash. Better to spend a little bit of cash and be safe. I got all of my glass for free on that project so when I threw it in the garbage the only thing that made me mad was all the waisted time and silicone.