We've used the large size binder clips in the past for BP tubs (between 4 and 6 per tub depending on tub size). Colin makes a good point though that you have to make sure they really fit the tub and lid. I've snapped one on to find it only grabbed the lid and missed the tub lip completely which did allow a snake to escape (only 1 out of 6 clips was this way and she found it!).

For a boa, considering their power even as young ones, I'd go with a more secure option like a very tight bungee cord or metal C clamps.