Here is the design that i used for my hopper box. I designed this myself, and you are free to use it and change it how ever you like.  The current design uses a industrial air conditioner vent as the grate, but you may need to find something else or design something yourself as a grate.  The air conditioner vent will get blocked by some rocks and can be tough to get them out.  The box itself works great though.  I think I changed the opening...