It's a more antiquated ad type but I wouldn't go as far to say it's a "tough medium to make money". If you can source the popunders for cheap enough, make a creative that gets enough interest(ctr), and find some product/offer/service that converts you'll probably see some return. For the creative, think something generic that might appeal to everyone(you're today's _____.com selected visitor! or You've Been Selected From The USERSGEOLOCATED Area To Test ____) and also test using sound clips that will get their attention. I've had success pushing different things off popunders, try linkbucks.com/adonion.com/popcash.net/popads.net or you could contact webmasters directly and negotiate a fixed price per month and have them add your popunder code.