Brian - I notice you said you used "tupperware or sandwich containers." I suspect these have tighter-fitting lids than do the deli cups that I use. Wonder if that might have been a contributing factor? Or maybe you cut an unusually small hole in the top?
I recently broke down and purchased the deli cup tops with the precut holes. Although I feel monetarily and environmentally wasteful for doing so, it sure is convenient. These have a 1-3/4” dia.. hole in a 4-1/2” dia. lid. I’d prefer a slightly larger hole but this arrangement seems to work okay.
Thanks for sharing your experience. I guess I won’t say “sorry for your loss” or anything like that. I, too, think it was a rare occurance, especially considering how long a snake can hold its breath.