Use tannalised timber, about 4" x 3/4". You can but the lengths in any DIY place for only a couple of quid for a 6' length. Nail it to 2"x1" (tannalised) posts about a foot long, use galvanised nails that will go all the way through and can be bent over. 3" should do it.
It lasts for ages. I edged all my lawns 10 years ago when we first moved in here and its still going strong. Eventually the wood dulls down as well, so you hardly see it.