Commodities are not meant to bought and held long term. Just pull up a long term chart of the Goldman Sachs Commodity Index chart. It ain't pretty. And that's not because commodities always go down, it because of the cost associated with holding any physical long term whether it be copper, gold, corn or oil.
There is no direct instrument. The actual index pricing is derived from USO options. There is no way for you to capture the 30 day fixed term structure. You can buy ATM USO options but you can't effectively isolate the vol.