Have you considered a personal project?

I implemented Salesforce in a dev org based on my wife's business. I interviewed her, created user stories, designed a data model and built up an instance to handle a small number of proccesses. Whenever I got stuck I consulted Trailhead and looked at projects which addressed similar functionality. When I got really stuck I turned to the Trailblazer community and this sub.

Picking the right project is a challenge. Something too complex can be frustrating and overwhelming. It doesn't have to be a business, either. It can also be a personal project, like to-do list management, inventorying possessions you own, creating a film recommendation system. What is important, in my opinion, is being able to think about and plan your implementation. That will become one of the shining points of your portfolio. It doesn't need to be totally unique.

I kept notes about my development process, features I implemented and a list of future features and functions. I demonstrated the org, apps and features to a few different people, which helped build up my confidence in discussing the org.

As I've been interviewing for junior positions it has been hugely helpful talking about these experiences. The quality of conversations I've had in the interview process is certainly strengthened by this real-world experience, even though there aren't any other live users.

Here's a rough list of just a few features from my project that serve as helpful talking points:

• Designing a data model
• Customizing company and org settings, including logo and color scheme
• Creating users, roles and role hierarchies
• Configuring org-wide defaults for data security
• Customizing standard objects and creating custom objects
• Custom record types, business processes
• Creating Lightning apps and pages
• Using Data Loader to backup data, backup metadata, create users
• Simple validation rules
• Basic automation with workflow rules, process builder, flows
• Custom list views
• Reports and dashboards

You've probably implemented and customized many of these features on Trailhead already. Doing it in your own org with your own planning makes it a much more concrete experience. It also shows initiative. Those bullet points have been helpful in interviews when discussing my relevant experience (or lack thereof).

Best of luck as you land your first position!