Cob: patio walls?

[Sarah Kopp]

We want to build a sukkah porch - the floor slab exists and we want to build
walls about 210 cms high that will hold a lightweight grid (woven branches)
to support the "schach" - the green branches that transform the whole thing
into a sukkah one week out of the year.

Has anyone successfully built outdoor cob walls - garden walls or the like?
I am wondering if it is feasible to build cob walls and cap them with tar
paper before plastering so the rain just washes down the sides rather than
soaking in from the top.  Plus maybe a good silicone sealer?  The slab is
elevated on pilings and not engineered to support the weight of stone walls,
so I don't know what my other choices would be than cinderblocks or pumice
blocks.  It is on the leeward side of a two-story house.

Sarah
Tsfat, Israel