Just to let everyone know there are two different types of Mud duck. First the original which is the design from the AMA plan source The second is from the web sight previously listed of Mud duck Aviation . These are built from coroplast or foam board and are very large. The only thing they really do have in common other than similarity in shape is .Yes same guy designed them
To give you some more detail, the BIGGEST thing they have in common is the shape -- but that IS most of what makes an airplane what is. Both the AMA-published balsa Duck and the foam-core giant kit duck look like a duck and fly like a duck... but the plans are very much NOT interchangeable.
The construction methods and the size are what's worlds apart. So you can't build the foamcore type from the built-up type plans or vice versa (Other than by designing all of the material specific details yourself -- but that's not really building from the plans... that's designing.) And you can't build a Giant Duck from the built-up plans... they're designed very light and would need the structure revisited to be able to be built big (which is designing again.)