I recently made a new lid for my smaller aquarium, out of clear panels of lexan (polycarbonate) that I got cut to fit at the hardware store. Even though it's not perfect (the cut was not right so I cut again at home with fine-tooth saw and tried to get it smooth with sandpaper but it has scratches on the edge now) I'm pretty happy with it. I followed examples found here. Basically it's two panes (most people use glass) that fit into this plastic piece meant to edge tile. One sits in the channel, the other rests on top, and to open you just slide one back. The op recommended having the bottom pane slide to open but after a few weeks of using this I found I prefer to slide the top pane, from the front. It sticks less.