I have Tetra 5.5 gallons which has an outside dimension of 16 x 8. I have several two part lids with a stick on hook handle that are made from a thick frosted plexiglass. They are very sturdy. Despite being frosted they let plenty of light in. They will be fine for a regular tank but may not work for an NPT. Some are just a smidgen off in fitting the lip vertically as they curled inward just a little. It does not detract from appearance or function of keeping your fish from jumping out. I have bought double strength glass as I intend to eventually go to all NPT tanks. I'd sell them for $3.00 each plus shipping. I have about 12 of them. If you have the right size tank and are tired of Saran Wrap or have nothing this is a good budget cover that will keep your fish in and prevent evaporation.
Here are pics of one the lids in use: