Try reaching out to professors or research labs at universities. I work with a professor who would love to hire more Software Engineers with industry experience. It is by far the most meaningful and interesting work I've done in my career.

There are tons of interesting projects going on within academia that unfortunately don't take advantage of any software engineering best practices. Many projects are prototypes hacked together in MATLAB and I know in our lab we are looking to build more robust products that are open source and extensible for other researchers to use.