Your approach using corrugated, bolt-together marine aluminum plates in a "box culvert" form factor is technically sound for a seastead superstructure. Key advantages you've correctly identified:
The "hard points" and internal frame you describe are the most crucial part of the design. An engineer must analyze:
You need a firm that combines marine fabrication with structural/civil engineering expertise. You are correct that this is not a standard culvert job. Target these types of companies:
Examples: Companies that build aluminum patrol boats, research vessels, ferry superstructures, or offshore platform components.
Why they're a fit: They understand marine dynamics, classification society rules (ABS, DNV, Lloyd's), and work daily with large aluminum structures. They often have in-house naval architects and engineers.
How to approach them: Present your project as a "custom aluminum modular marine habitat." Be prepared with preliminary sketches and your load assumptions.
Examples: Major distributors or mills of marine aluminum (like Hydro, Novelis, Kaiser Aluminum) sometimes have partner engineering firms or can recommend specialist fabricators.
Why they're a fit: They have deep material knowledge and industry connections. While they may not do the engineering directly, they can be a gateway to the right partners.
Examples: