We had the same issue at our venue and installed mobile phone charging locker about six months ago. Customers secure their phones while charging instead of sitting by wall outlets or carrying cables around. The lockers accept payment and people get a code to retrieve their device when charged. Reduces complaints significantly and customers appreciate having a safe place to charge. Also freed up seating areas near outlets since people don't camp there anymore. Initial cost seemed high but pays for itself through customer satisfaction and they generate some revenue too. Way better than just adding more wall outlets which creates other problems