I kept the cups my bettas came in, they are a good size for taking water in and out during changes. As such, the bettas seem to be used to seeing them, and don't hide. I've only taken one completely out of his tank once so far, when we were doing a complete tank revamp, and in that case I just put the cup in near him, and scooped him right up. Maybe a smaller cup would work to capture him? You could then add him to the bigger bucket of water scooped from his tank.