use a cake batter consistency cement and fines slurry, apply a few mm layer to the foam with a trowel/wide paint scraper. Push in your hardware cloth or fibreglass after and smooth over. Allow to partially cure, then build up your tufa layer on that. Put separate pieces on each side - don't try to fold around the corner.

Alternatively, if the post isn't very tall, just 'throw' handfuls of sticky tufa mix to the foam and it will bond quite well.

The mesh is to stiffen the post, not reinforce the tufa - if the tufa can't hold itself together without mesh, it's not got enough cement in it.

I helped my grandson make a short faux bois "column" a month or so ago using a similar strategy. He formed it over two stacked gallon pots with crumpled newspaper as spacers between the pots and the hardware cloth he used as a support for the base. Then he pushed on concrete mix (not hypertufa) through the hardware cloth, smoothed it, put a layer of mesh over that and then applied the final layer of concrete which he then "carved" to look like an old stump. When it was set up two days later, he pulled out the newspaper and then the pots and had a base for his plant stand. He then cured it by wrapping it in a sopping towel enclosed in a black plastic bag for a month, spraying the towel when/if needed. You can see the result at the link. Hope this helps.

Here is a link that might be useful: Concrete stump