Really try to use plastic canvas; it will produce much higher results at your current size, than trying to get another piece of plexi. Info:
Try going to hardware stores, craft stores, garden stores, sewing stores, or just get one of these online:
This "plastic canvas" one might be easier to get into the slot after cleaning, and the edges will not wear; it also will hold it's shape so that a solid frame may not be needed:
http://www.everythingplasticcanvas.com/pc-20049-260-plastic-canvas-7-mesh-12-x-18.aspx
This "rug canvas" is made from fiber so that algae sticks to it the best; but it does not hold its shape when wet, so it will need a solid frame. Also, the edges will wear, so the frame will have to hold the edges down:
http://www.joann.com/joann/catalog.jsp?CATID=cat2099&PRODID=prd12195
This "tank divider" is mentioned here because you can get it in any LFS, and because they come with clip-on edges that are great frames for any screen. However the screen material itself is very thin and smooth (needs sanding) with not many holes, thus during cleaning it's hard to get algae to stay on the screen:
http://www.aquaticeco.com/subcategories/2/Tank-Dividers/tank divider/0
Overall the rug canvas will grow the most algae the quickest, but takes the most work to keep it in place. The tank divider is the easiest and most available, but you'll get a few days of no filtering after cleanings. I think the plastic canvas is best for most people; it holds algae on the screen after cleanings, and it holds it's shape when wet and when cleaning in the sink.
Many people ask about using the fiberglass screen from their windows (never use metal!). The main problem of this kind of "soft" screen will be getting it into the slot in the waterfall pipe; it will bend and fold too much. One way around this is to loop it around the waterfall pipe and attach it to itself, instead of slipping it into a slot. In this case you don't need a slot; a series of holes will work. But this screen door method is only a last resort though, and will not work that well long-term, because the algae will start blocking flow out of the pipe since it's wrapped around it (it will need to always be trimmed.)
