Yeah the outdoor ones definitely look like cement. The indoor ones are probably too. Though they have a patina that makes them look like worn iron.

Lynch made those himself out of grey plaster. They didn't show much of Lynch's studio, but that's where he spent most of his time hanging out, making sculptures and paintings and building things. He was hands on guy who kept himself busy, compulsively so.

Maybe they would catch rainwater too - mosquito breading area