I think there is an old thread about playing your instrument on the toilet & bathroom acoustics but have you ever tried the outhouse? I am told it is amazing!
This weekend at a cabin with some friends, one of the guys spent most of the time playing his guitar in the outhouse. He said the vibration from the plywood walls had so much to offer. That it was just like being inside a guitar.
Personally I think the smell would interfere with my creativity plus the flies can be quite distracting. Maybe I should try playing my banjo on a bidet? I bet all that rushing water would offer background nature sounds. Why I could start a new line of banjo solitude music.

Little Neo