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[cris]

I like using cardboard boxes and non seethru plastic containers as mentioned use the smallest possible. I also use cardboard six pack holders that have been flattened this creates a very narrow gap that will adjust to the snakes size.
If u want a natural look split a hollow log into 2 or more pieces and sit it on the ground or on a branch in a way that it cant fall. You can also use hollow logs but keep in mind u may not be able to get the snake out, but u can just move them in the log if u need to.
you can also use a number of rocks to construct a cave like structure.