That's a well-qualified response. I'm more accustomed to working with model designs from software, rather than a real-world object and neglected to consider your point.

The only method I can fathom would be SLS nylon printing. It's astonishingly strong, but equally astonishingly expensive and typically used for smaller model creation. Something the size of a seat pan would be really expensive.

If you can create a scanned model in software of the desired seat pan, you may be able to find a hobby CNC owner willing to carve out an injection mold. For CNC owners, the harder part is the model creation. Once that is in hand, the machine does the hard work. Not cheap, I'm sure, but still an option.