You may be closer to a portfolio than you think. A portfolio is no more or less than documentation of what you have done. Even if it is just old projects and scripts on old computers, you can still write it up -- Explain the problems you solved, the tech you used, some notes on the success of the solution, screenshots of the UI if possible. Put it all up on a web page and you have a portfolio.

After all, portfolios are just like resumes in that their purpose is to grab the attention of someone to get a conversation started. Focus on telling a story that makes people want to hear more.