I've used 1/4" in most of mine, and no problems. I used it as a matter of convenience and what I had on-hand. If I had 1/2", I would have used that. If buying new and it doesn't create any additional work, I might opt for 1/2", not that much difference in price here between a sheet of 1/4" or 1/2". If you try 1/4" and it sags, you could always replace it with 1/2" or put a stiffener under the bottom.