Update:
Someone came up with a great way to attach Rug Canvas. Rug Canvas is the highest performance screen material; it is preferred over Plastic Canvas because Rug has small fibers that algae can attach tightly too. This means you get growth faster, and you get more growth sticking to the screen after cleaning (thus, no overly-cleaned bare spots). The problem with Rug is that it's a flimsy material, and the edges tend to unravel. It also won't last forever. So consider it more work, in order to get the highest performance.
Anyway, this idea is very simple, but I've not tried it. So you might have a plastic canvas version as a backup, in case you can't get the Rug working properly. You'll need to make the slot wider, to accomodate the plastic rod. The trick will be getting the right "fit" between the rod/screen, and the slot, so that the water flows smoothly. It will be trickier than a simple plasic canvas, no doubt. So plan on experimenting with it for a few days in the bathtub.
You can get Rug Canvas at any crafts/sewing store. Also, you might need to sew/glue/hotmelt/etc the loose edges so that it does not unravel.