Ongoing/undefined opportunity to help with our team's corporate assets

We need a designer that can be available on-demand, and that can help our team make presentations and content presentable to the Board, to our customers, to our prospects, etc. The characteristics about this job/project are: 1. there is no need to create new designs as we have plenty of templates and brand guidelines to be followed; 2. we need someone that has relatively short response times; 3. we need someone on an ongoing basis, where the amount of work will vary based on demand.