My previous UX work was based on research and usability testing, and
unfortunately I no longer have access to this work or have kept
samples of it.
You can still reference any work you have done in the past as long as its not covered by Non disclosure Agreement and your work experience is verifiable.
Any advice for creating a portfolio from scratch and what to include
Your Research portfolio should focus on:
Demonstrating knowledge of different methodologies : The idea is to
show employers which method you opted for in specific contexts
(Quantitative/Qualitative) or more focused on (attitudes/Behaviour)
Demonstrating the process used to define research goals: how you
came-up with your primary question. what other questions needed to be
formulated in order to answer your primary question.
Assumptions and hypothesis: How you derived these and what type of
information you gathered to validate them: the focus here is on the
type of data/information gathered (testing) and how they helped
confirm or refute assumptions.
You could also conduct your own research in a subject of interest to demonstrate the above which will help convey your know-how and skills.
Also what medium should I use to create the portfolio itself?
The medium is less important here as you are more focused on research compared to other UX deliverables ( knowledge of wire framing tools etc.)
That being said, in principle your choice should help you illustrate your thought process and demonstrate your communication skills such as the ability to translate, synthesise requirements and findings into easily digestible chunks of information or conceptual models that other team members or stakeholders could use.
Hope that helps