Blast, from what I've read you probably want to grab at least one more identical bin to make a spare inner bin. The outer bin is mostly to catch any effluent that may drain out, I doubt much of the castings will sift through into it.

Instead, when the first inner bin is about full of castings, add the second inner bin on top of it and start adding food and bedding. Eventually most of the worms will migrate into the new bin and you can remove the first inner bin to harvest the castings.