"You could probably MacGuyver one for even cheaper..."
Agreed.
You can buy one sheet of craft canvas, that's the plastic mesh screen type stuff that can be found at WalMart in the craft/fabric/yarn/needlepoint section, as well as craft stores, a package of four small suction cups, a tube of aquarium safe silicone (some have used super glue with no troubles and it's commonly used in reef tanks but I've never personally used it) and you would be set.
Just cut the canvas to a size that will fit the tank allowing room on both sides for the suction cups, silicone the cups to the edges (if you find the right suction cups you will be able to slice into the "post" end and fit that onto the canvas before siliconing it into place) wait 24 hours for it to cure and you are done.
The craft canvas comes in many different colors including a semi "clear" one. Just make sure the suction cups are not so big that they will allow too much of a gap which would let the fish pass back and forth between the edge of the canvas and the glass. You may even want to cut the canvas so that the area the cups are attached are "bumped in" a bit. That would allow the canvas between the cups to be wider and leave less chance of the fish visiting eachother.
