This is not the sort of thing I’m equipped to try, for a variety of reasons, but Mikey Sklar and his girlfriend Wendy were looking to build an enclosure for their PV battery bank. They eventually settled on a dome of rebar, remesh and papercrete. His Instructable details the process, and he’s also posted a video to YouTube.
It’s cool enough that Mikey’s dome houses the guts of a PV array, but I’m looking at that picture and thinking "playhouse." Anyone willing to take a stab at that? Your geeklet could be the coolest kid on the block with his or her own papercrete hobbit hole.