this is one of my favorite questions in the Ryan Kulp FAQ. whoever asked it, well played.
while chatting with my friend Youssef, we both agreed designers have it best when it comes to building a portfolio.
as a web designer you can visit a URL, imagine a new look and feel, then send a screenshot of your mock directly to the person in charge of paying designers. if you "wow" them, you'll probably get a gig.
as marketers, it's not so easy. the closest match would be researching a company, reverse-engineering their growth channels, then presenting ideas to either optimize an existing setup or "wow" them with new ideas.
but the "new ideas" part is difficult, because what might seem like a great idea to you could actually be something they tried a long time ago, and it didn't work.
on more than one occasion i thought i had a brilliant idea for a company i wanted to work with, only to find out they already executed that exact idea. #facepalm
to answer your question, amateur marketers need to create a portfolio project that does not rely on someone else's hidden insights and does not require gatekeeper "approval."
this means, perhaps, that the best amateur marketing portfolio does not speak to companies. instead, it speaks to the individual creator behind it.
for example, instead of trying to score a case study with a company that is unlikely to hire amateurs, write a piece like How I Would Growth Hack a Barbershop.
demonstrate you're creative and the gigs will follow. then you can turn those gigs into case studies that prove your creativity can be applied for profit.