They use gourds as water containers. Cut one in half and soak it in a bucket for a while to see what happens. If it passes that test the only other question would be regarding if it leached bad stuff into the water. Have you looked into coconut huts?? You can DIY them pretty easily.
I think the Gourds that are used for water jugs have been coated in something...we have gourds growing outside and they are left to dry there but when it rains they become mushy.
I think a better suited option is coconut huts like daFrimster stated or tiny terra pots from your pottery store.
check out Cichlidbreeding.com for some really nice looking caves.
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