I've always gone with whatever size will wrap around tightly and have no slack. I prefer the flat bungees just for how they sit, but the round ones work just as well. By "lip to lip" I assume you mean across the top and hooking on the opposite lip? I was never confident that it would hold against a determined boa like that so I went with the wrap around method. Never used clips but then ocne they get to be about 3.5 to 4 feet it's time that they are moved into an appropriate enclosure that is designed for them, at least in my opinion.