First, you'll need to get a few things:
1. Any color report binders. You'll need at least 2 to divide your tank in half. I found a pack of 5 of these for $3.97. I prefer black but any color will do.
2. Craft canvas. I didn't know what this was. It's flexible thin plastic with holes in it. It comes in a variety of colors and you can find it in the fabric section at a craft store. I chose clear so my bettas could see and flare at each other. Black is a good choice if you don't want them to see each other. Again, it's up to you. I paid $0.97 for a sheet.
3. Aquarium sealant. I got mine from Home Depot for around $4.00. It's non-toxic and should be the ONLY thing you ever use to glue/fix your tanks with. Good thing to have around just in case.
That's it for supplies. You should be able to get everything you need for about $10.00
Okay. Now to the fun part. And I'm serious. I had a good time doing this.
1. Rinse and dry the tank well.
2. Measure the height of your tank.
3. Cut 2 report binders to the length of above measurement. Your binders should come above the water line. But they should fall below the top of the tank so you can have a lid.
4. Take the aquarium sealant and squirt a line down the solid side of the binder.
5. Stick this directly onto the aquarium. Hold it for a few minutes.
The sealant says to let sit for 24 hours.
6. The next day when it's dry, slide your craft canvas in the open slots in the binder.
7. Rinse your tank well.
8. Your done!!!
The plastic canvas doesn't move out of the report binders. It's pretty sturdy stuff.