Just start with some plywood or any junk wood, check garbage day’s people throw out old dressers etc the bottoms of drawers work well and that junks free! lol
Then cut it to the size you want.
Next add the plaster, I use regular drywall compound, you don’t want it very thick just enough to give a surface. If you put it on thick it could warp the wood
Once dry, sand it down to even it out, and use a 220 then 600 that way you don’t end up with scratches in your finished surface.
Then I like to add the cuts that most concrete has. I make 3 inch squares, that seems to be a good size scale wise.
Then just paint and weather how ever you want!
