Clear CD case keeps wasps off my hummingbird feeder

Last summer the wasps took over my hummingbird feeder in about a week. They'd swarm the ports and scare the hummers off, I tried taking it down and cleaning it with soapy water but they came right back. Then a guy at the feed store in Eugene told me to slip a clear CD case over the feeder body, leaving the ports sticking out. The wasps couldn't figure out the angle to land on the slick plastic, and the hummers didn't care at all. It's been 3 months now and I haven't had a single wasp on it, just a few confused yellowjackets buzzing around the edges. Anyone else deal with wasp issues on their feeders, and what's your fix that actually lasts?