The jute rope technique sounds wonderful and keep us posted on how that goes. Also, if this prop is going to be displayed outside, specifically with regards to moisture, I suggest you seal it well with a product called Sculpt-or-Coat. Since this product leaves a bit of a sheen, I would also use another product called Flat Plastic Varnish over the Sculpt-or-Coat. Both of these products can be purchased here
Foam Coatings, Clear coats, Primers, Saturated Paints, Scenic Brushes by Sculptural Arts Coating, Inc. and they last a very long time. I use it on every mache prop I sculpt even though they're not exposed to the weather.
He really is a beautiful piece of work and I'm very proud of you.