Which overflow do you have? The problem you will find with putting anything around your overflow to stop fish from entering it is it will probably prohibit flow some and therefore you may run into the problem of your pump being too much for your overflow to handle. I tried it once with some fishing line, weaving it through my overflow teeth and ended up almost draining my sump :shock: Was just too much restriction. I ended up switching to a CPR overflow which is pretty good at keeping the small fish out.
Here is a few pictures of mine.
I just cut it to fit the opening and split a few squares to hang onto the glass and keep it in place and then you can see from the top view, it sits under the frame of the tank.
Well I think I got it at ACE but I figured you could get it any ware.
Sorry about that.
Heck I have a 25ft roll of it you could cut what you need from if you were in the area.