While it's not much to go on, the work that had already been done on Eastport and the fact that iron had been collected to armor her is a pointer... and I believe rebelatsea extrapolated from what we know about Confederate conversions like the River Defense Fleet gunboats and the design of the Arkansas (please correct me if I have that wrong, John).
On the flip side, the conversion design as completed by the Union bears some strong similarities with the USS Lafayette, under conversion from the Aleck Scott at about the same time (though without the fancy curved frames of the Lafayette); I wouldn't be surprised if the Lafayette's conversion affected the course of Eastport's. (Or vice versa, I suppose, though I think that William D. Porter was more apt to go his way than to copy what Phelps was doing...)