This site provides a good framework for students as they think about about what they could and should add to their portfolio. There is a strong self-reflective component built in. This also is pre-digital but it could be easily incorporated into our digital platform.
I've only read the first 15 pages of this long article in pdf, but the first two chapters seem very useful in showing how to use portfolios in the math classroom.
The latter half of this pdf is full of excellent resources and ideas for leveraging portfolios in math.