Okay, okay. I’ve got an idea.
It’s not actually there. You just make people think it’s there. Signs every 50 feet or so that say INVISIBLE DEATH BEAM WALL! TRESPASSERS WILL BE INCINERATED!!1! !!
Then scatter tons of charred bird corpses and the occasional partially incinerated human cremains from poor people institutions like prisons and cheap old folks homes along the way.
I figure it’ll be as effective as an actual wall, because hey, it’s not like people could tunnel under my dead bird idea either, right? And my idea has the benefit of being both cheaper and far more metal in concept. I’d really prefer rich bastard cremains, but those are wayyyyyyyy more expensive.