There are many varieties of worm bin designs, but my favorite is the indoor worm bag design
. I made my own before I'd seen that design, so until I get a big increase in the amount of food waste my house produces, I don't think I'll make another one.
My bin is just a plastic storage bin with some air and water vents added. The main vent is a length of thin-walled PVC pipe that runs the length of the bin. I first drilled small holes along the pipe, then cut holes on either end of the bin with a pocket knife. The pipe is held in place with Gorilla Glue. There are also some smaller vents at the top of the bin that were there when I got it
Where you see the red electrical tape is where I tried putting window screen over the vents to keep flies out. It worked for the most part, but some of the smaller flies were still able to get in and out.
You can also see my backyard compost pile, which sits in the corner of my chain-link fence. It's not much more than a pile, although I have piled up some lumber on the third side, and added some window screen to keep mice from burrowing into the pile.
I've made this into somewhat of a "worm bin annex," adding worms to the pile, and putting compost into the worm bin to be finished.