I'm thinking of two big old concrete walls about 50 yards apart, with a mined "no man's land" in between and razor wire up top. Oh, and devices underground that can detect tunneling.
This would be ideal for the US-Mexico border. Might give some of the illegals a second thought...
No doubt that would work. However, what about the beaches and all those expensive beach front properties? We have difficulty getting political support for much less extensive measures. After all, the Dems see the illegals as probable voters and a reliable disenfranchised welfare constituency so they are for it.
Those in the concrete business would love it, though.