Oil would work. It has a large and liquid market. And they have plenty of it.

How would you store oil if you wanted to actually have possession of the asset? Barrels of oil in the basement seems pretty unreasonable.

The same way you store gold: You pay someone to store it for you. It's bigger so it's harder to store, but it's also harder to steal, so it needs less guarding.

I'm not suggesting it as currency, just if you need something to back your currency oil works a lot better than gold.