Hello Bill,
Sorry for so late a reply but I've just joined recently and only decided to rear my ugly head today. I don't know if you've been able to come up with any plans or drawings for the Cairo but 2 decades ago I built the Carondelet (a sister ship of the Cairo, in fact they all looked the same. The navy had to paint the smokestacks different colors so the sailors wouldn't end up on the wrong ship) based upon an article in a scale modeling magazine. I cannot for the life of me remember which magazine or which issue but it may have been, possibly Scale Ship Modeler or Model Boats from the early 80's. I was able to get a copy of the Historic Structure Report which contains a set of lines but no dimensions. The report does show EVERY timber that was used in it's construction as well as detailed drawings of the engines, boilers, guns, armour, and more. These drawings along with the article helped me to build a decent model of the boat. If I wasn't interested in building R/C boats I would seriously consider building a scale half hull cut away model of her.