@sun In the past I've used temp (labour) agencies to get quick work and cash between regular jobs. Depending on the agency and work some will pay you cash at the end of the day, provide transportation to the job site and basic tools and clothing. A couple places got to know me as reliable and a good worker and would bump me to the head of job lists. Some companies use temp agency labour to filter for possible future full time employment at the company. I was asked twice.

The work can be not great and is physical but it's not back breaking. And I liked the variety in the work.

Another thing with your STEM background is tutoring high school and college level courses. For high school especially it's easy to bone up on math, physics and chemistry and there is always a demand. Easy to get cash payments as well.