Forget the Fence...

Let's go with heavily armed guard towers every 100 yards:

Men in Mexican military-style uniforms crossed the Rio Grande into the United States on a marijuana-smuggling foray, leading to an armed confrontation with Texas law officers, authorities said Tuesday. No shots were fired.

Just to take the human factor (racism, greed, corruptible) out of the mix, lets arm each guard tower with, say a M242 Bushmaster 25mm Automatic Gun hooked up to some sort of motion detector. Sure, some tumble weeds and vermin (small furry creatures, terrorist, drug smugglers) might get wasted, but NOTHING is coming across the border.