My only suggestions would be trip wires, fences, electricity, the occasional sneak attack at night with a cattle prod, getting on the roof and setting off LOUD firecrackers, leaving out tainted food (egg salad sandwiches with lots of mayo that have "fermented" in the afternoon sun), etc. What are the chances of retaliation from the Mexican "tourists"?
To tell the truth, I suspect the mayo could be the culprit in some alien deaths. Seriously, we always seem to find a couple of mayo jars abandoned at load out sites. Having a salmonella sandwich when you're already hovering on the edge of dehydration is not wise. One good bout of Montezuma's Revenge could put you in Death's shadow.
A lot of the illegals come from Mexico's tropical or semi-tropical regions and don't realize the hidden danger of Arizona's patented Dry Heat. You can be sweating like a horse, losing fluids and electrolytes like crazy, but not realize it because your sweat is evaporating as fast as it reaches the surface of your skin. You don't know you're in trouble until you're really in trouble and then it may be too late. Various body functions just start shutting down and without medical intervention you end up as just one more mummified puzzle for the coroner.