I found some useful references:
General info: https://www.structuremag.org/article/shipping-container-design/
Top Rail dimensions: https://containersurveyor.com/what-you-should-know-about-top-side-rail/
Many years ago I looked into sea-can structures for a bit, from memory I think the roof could support ~30psf.
In their normal use case most of the load is transferred through the columns in the corners. I would be concerned if your bearing line doesn't align with the wall of the sea-can
