Rendering a herb spiral

So I've build a 2m by 2m herb spiral and filled with soil. In the process of planting it. However. I used reclaimed materials for the walls - bricks, concrete blocks, chunks of granite, stones. This has given it a very rough look and some of the stones are loose. I am wanting to render/clad the walls to strengthen it and neaten it up. My thoughts were to wrap it in chicken wire or mesh and then clad with a mud/clay mixture. do it on a sunny weekend and allow it to bake dry. Obviously whatever I use has to be reasonably weatherproof (I live in midland England so can be rainy) but it's also important to me that whatever I use does not add toxicity to the soil or leech anything harmful (or even drastically change the p.h). Could anyone advise a solution? Thanks - Samuel