I was thinking the very same myself... wallpaper could be good...
In the UK right now there are some fab designs - some very retro 1960s copies, which could look cool, or even some which look like leather, in cream or brown.
I wondered about cheapo masonite (hardboard) for the inner ceiling, papered with cream leather wallpaper, and varnished ply for the walls.
If I was going to use wallpaper, I was considering fixing it with PVA glue rather than wallpaper adhesive, since PVA has a kind of waterproofing quality.
