I have an idea: the Hunting Shack!
It has 3x3 interior, and can be built (excluding deep water) anywhere.
That means swamp, forest, pretty much anywhere.
It would require:
(edit)50 Blocks of Wood
25 Boards
25 of any bough
1 rope (to get INTO the hunting shack)
It would look like a log cabin, except smaller and on short stilts.
It would require Carpentry and Hunting.
You cannot carry containers or other furniture in, however you can build containers and such inside it.
You cannot make it have a cellar.
Entering it disables creature aggro, but takes 5 seconds.