Don't do it. The usefulness of such "portfolio" to your ability to obtain future employment is essentially zero. And your tendency to give away your employer's property
to the public, for poorly though-out reasons to boot, will be a big red flag to your future employers.
You have your CV/Resume, you have your references. Since you were not fired from your last job after 3 months, every reasonable hiring manager will safely assume
that you indeed have the skills and the experience that you listed on your Resume.
If they need a further evaluation, they will find a way to test your skills in a way that benefits their unique situation. Analyzing somebody's old code is usually not it.
If your interviewer has specific questions regarding technical aspects of your previous job - impress them with your good memory, good technical knowledge
and excellent communication skills - by quickly and intelligibly summarizing your achievements on a white board.
Nobody will have time to look at the source code that "fell out of the back of a truck" of your former employer.
Edit: includes an answer to Tonsteri question below.
The application forms you are filling are generic forms to serve for all types of candidates. If someone was working on Open Source projects, or is an independent consultant and shares his solutions with the public as means of self-promotion, or worked for a company that encourages "tinkering on the side",
then they have a "portfolio". However if you spend 10h a day doing proprietary UML designs and writing proprietary code for Big Pharma, then obviously there is
no public "portfolio".
A competent hiring manager knows all this (and trust my 20 years as a Solutions Architect and Software Engineer: you don't want to work as a junior-to-mid-level
personnel for people who don't know what they are doing). The worst thing you can do is having a "forced" portfolio made of your employer's code, where
no reasonable manager would expect you to have one. It shows a lack of common sense and poor judgement. If you want to have a real portfolio - get involved in
some Open Source project, or just create some libraries that provide useful functionality that people ask for online.