When Coby was little and would "wander off" when I knew he had to pee, we would either use a leash to keep him on the pad until he pottied or we'd use a small pen just around the pad that he'd stay in until he pottied (sounds exactly like the big bin you're using). Before long, they'll know that pottying on the pad is what gets the good treats. And they learn that play time comes AFTER pottying.
Mine now can pretty much potty on the pad on command if I know they need to go. And as little Coby will tell you - it's the law that a treat is given each and every time you potty on the pad - so both he and Reggie come and tell us when they've pottied on their pad.
