I understand where you are coming from! If I am correct, your concern is more to do with 'how' can you show your research in a portfolio and not so much with the actual process of how to do the research. Hence I will aim my answer at that.
1) Consider "Impact Maps" which have worked great for me, both in getting requirements clear and often to fuel great discussions with users/stakeholders/team members.
Here's a good video to quickly grasp the same: https://www.youtube.com/watch?v=RznIi2WkJb0
2) Once you do such an exercise, you will have a tangible diagram/chart of sorts to include it in the portfolio.
3) Taking that as the starting point, you can possibly prepare a "User Story Map" which is a great activity/idea/method to visualise the whole project at once.
Some links for reference here:
There's more you can do in form of sketchy Personas, Journey Maps etc - but the above two have worked out great for me, both in terms of fuelling users to add inputs and then taking that input to form a product plan. The idea should be to showcase the effort in making the research, and not the decorated final template/artifact. As I said, since there is a 'real' tangible output, you can showcase the same in your portfolio.
In case of minimum access to users, and as a project - you could do this by yourself, considering the various scenarios/users/behaviors/goals. Obviously, any real user involvement would be great. Important is to keep a positive intent to solve the problem at hand, rather than producing these charts/diagrams/outputs to just adorn the portfolio.
Hope that helps!