Danl,
do a search for VSSD < vans super secret scenic dope> I've posted a couple of recipes on here, for varying applications. As stated above you could manufacture the armature from PVC or sonatube or carpet tube, as long as it's realtively sturdy, then an application of VSSD and you can scuplt/coat all the surfaces to be as realistic as you want. The nice thing about VSSD is that it's realatively non-toxic, as it's all latex based, it cleans up well, and depending on which formula you use, you can make it extremely hard, slighty pliable, water resistant, and all the surfaces have decent "grip" to them. As I said search for the VSSD recipe, and play with it a bit, a little time expirementing will have good result. You can stretch the formulas by adding all sorts of things, as long as they are water based or soluble. I've used plaster mix, shredded newspaper, newspaper pulp < drain it real well first.> Water Putty, Vermiculite, sand,
etc., etc. Good luck.