looking good. An easier way would be to make your tufa in a shape that would contain it rather then making the tufa around the object unless you like the tactile feeling of making it yourself, which is fair enough :)

You can make this in three pieces. Use a damp sand mold to form the top and bottom then use a cardboard tube (carpeting stores have them. I think a sonotube is too big for your project). When they are partially cured cement them together and cover the whole thing with a slurry mix.
OR do a search on this forum and make yourself an armature. There's a lot of good info in this thread. Scroll down to the link for handsdirty "faux bois table link". Tango makes some great armatures but unfortunately his links are no longer available.

Here is a link that might be useful: armature discussion

Thanks guys! I do like the feel of molding it. And it wasn't that hard to do. I made the mixture a little thicker for the 36" tall stand. It did slump a little, but that was OK. It made the base thicker, which was good. I let it set overnight and just carved it with a utility knife and then scraped it with a wire brush. It looks amazing. This is tall slim base. When I made my thicker bases ( from concrete) I used sand and tube.
I also weighed the tufa compared to my other bases made in the same mold. The tufa was 1 1/2 lbs lighter!!