I would strongly advise using wood instead- I, for one, would fine it much easier. If you buy wood, you can have it cut at the lumberyard to size; the onyl tools you'd need would be sandpaper and glue, then. [You could bolt it together, but glue will probably do the trick.] I'd be more likely to collect branches, strip the bark, and let them dry out- then you don't have to buy wood [except possibly for the floors, which would be better flat] and the only additional tool you'd need would be a small hand saw.
Sorry I can't give you and papier mache tips.