If you're in the freelance content creation business and do most of your work through a third-party writing site, you've run into one of the biggest pitfalls that freelancers regularly encounter. How do you balance being under a contract that prohibits you from identifying the content as your creation, while showing prospective clients who might want to hire you the quality of your writing skills?
Create a personal blog that serves as a portfolio. If you use Wordpress, Blogger or one of the other free platforms out there, you have a ready-made solution that won't cost anything (unless you should decide to register a domain to point to your site, and that's fairly low-cost).
Write about your personal interests and topics where you're knowledgeable. Think of it as having your personal magazine to wow prospective clients with, without having to own a publishing company.
There's nothing to stop you from creating blog posts similar to content you've done for clients. For example, if you did content for a client featuring haunted locations in Savannah with a brief overview of each, why not do a series of posts with in-depth historic location on some of the places listed?
Using this technique, you won't be reusing content that now belongs to someone else. However, you will be highlighting what's best about your writing abilities.
Although most clients do expect style guide conformity, I'd wager that most also appreciate a spark of life and personality in your writing. After all, this will probably be the closest thing to a face-to-face interview you'll experience, so why not make the most of it?