Ah look, I found the pictures on my old website: I had a problem with a fish going behind the overflow cup in my tank.
Here are a couple of my first attempt, I just put some of the mesh screen around the edges. Of course he would just squeeze past it since I didn't want to attach anything to the back of the tank.
What ended up working was eggcrate behind the back of the filter so that he wouldn't want to be back there, and then a hard plastic along the outside. All of this was attached with hot glue. The plastic along the outside didn't work so well because this fish managed to squeeze through the tiniest crack, but along with the eggcrate managed to get him to give that up and go with something else...
...so where did he go, you ask? He tried swimming inside the overflow cup! A mesh screen around the top part of that kept him out, since it was easier to attach more securely. The downside to that is that it was right next to the light and I was battling algae at the time, so I had to scrub it with a toothbrush to keep the algae off a couple times per week, or else the water wouldn't flow as well.
Adam Horton is a decently common name. I wasn't the only Adam Horton at my college, and there are several on facebook. I got the adamhorton.com domain name, though