Best way to add buffer/contingency hours to a contract for unknown potential work

I have a contract where I am wanting to add some additional hours to the contract for things that I do not anticipate. This has to be included in the fixed monthly payment, so as-needed tickets only work if they are $0.00. So where can I store those hours? I don’t really want to fluff up other service hours for this, I’d like it to be its own line-item.