Real G
Too bad the major kit makers don't dip their toes into this untapped market.
The lack of documentation and/or physical examples probably deters them a bit.
Investing a hundred grand into moulds just to find there's an obvious flaw, that everyone notices right after the splashy kit release.
Also, much of the Union riverboat "fleet" was not built to plan at all, but ginned up in dozens, scores, of boatyards "to need" rather than "to spec."
Many of those were armored just by spiking in "salvaged" rail in alternating up/down configuration. That's not really an easy surface to model.
The joining edges were often left to run ragged, too. Again, not the ideal thing to mould for a model.
Eventually, plate steel replaced "improvised" iron cladding.
The CSN ships/boats were built to even fewer plans and specs. They were as often armored through heavy (12"/30cm) timber as any other material. Cotton bales, coal bags, all manner of things were used.
So, a person can, with a bit of gumption, simple scratch a boat-shaped base hull and give it a few rifles or cannon. There's a tough market to offer model kits to. You can scratch build to exactly fit your needs (your bookshelf) and is not some arbitrary scale nor size selected by the manufacturer.