Underhill uses a spring pole lathe on a lot of his shows. For portability I think you could make one of those that would be a fair bit lighter than the flywheel variety (although it would require a bit of setup/teardown). On the other hand I reckon a flywheel would be easier to use (or at least more similar to a modern lathe for using).
Shannon Rodgers has detailed his building of a treadle lathe on his blog and I believe his pay for content website. Also I believe that Bob Easton also built one on his blog. From these two bloggers, you can continue your journey.