I just got my hands on some bales of straw. They can be stacked or set upright to make them bigger. I also saw a barn that had a bunch of them in kind of a horseshoe shape to make a one stride or an in-and-out.
Go get some wooden pallets. You can cut them down to whatever height you want, nail some feet on them to keep them upright, and stuff them with artificial Xmas tree branches. They make wonderful brush boxes. If you use the cutoff portion for the feet and get thrown away tree parts....all it will cost you is the price of the nails and very little labor.