I fixed my retaining wall to a length of thin plywood on the work bench and then stood it up in place, I painted the plastic first and then glued it to the ply with UHU glue which bonds to almost anything and only a pound a tube in the local pound shop.I cut the plastic sheeting and buttresses to size and fixed them to the ply flat on the work bench which was a lot easier and more accurate than trying to fix them vertically on the layout.
when the wall was in place I fixed the railings along the top.
Hope this is useful to you Carl L
Puddles

It does not take me long to do five minutes work.
Nostalgia is not what it used to be.