For copy you don't have in final "pretty" form, you can simply put at the top of your page, "In Production." For copy that is written as a SPEC AD, meaning you wrote the ad on your own to show your writing ability, title it "SPEC AD."
Remember, employers viewing your copywriting portfolio are looking at your ability to write. They're not hiring a graphic designer so don't try to pretend you are one by wasting all of your time designing a mock-up ad so that your copy looks more finalized. Even seasoned copywriters put their best work in their portfolio and that doesn't always mean they have the final printed version of the material so they use plain text on paper.
To dress up those plain text ads, print them on professional-quality paper. This is the same type of paper you would use to print a resume on.
To really make that text ad stand out, buy a decorative background paper and offset the two by about an inch. The image above shows an example of how great your sample can look with just a little extra effort.