I have one in my 20L, which is the same floor space as a 29, but just less height. Uli's just about always on it, when she's not in her hide (she's on it right now, hehe).
As far as I can tell, they don't need a whole lot of floor space in order to get around and it makes the tank more interesting. Plus, if you build it yourself, you can be sure it doesn't take up too much space, or get too tall.
But if you do give them something to climb on, make sure your lid is on good and tight!
