What you do is buy two layers of mesh or plexiglass or acrylic, whatever you want to use as your dividers. Then put them in with a bit of space in between them. Just big enough for a piece of paper to slide in. Then choose a piece of paper (it can be colored, it can have a picture on it, anything you want really) and laminate it, so it doesn't get soaked. You can do this by covering it with clear packing tape, or actually laminating it. Then slide the piece of paper between your two "slices" of divider. That way you can give them a little flare time (great for pictures), then just slide the paper back in and give them chill time. It is great for bettas who constantly flare through the divider and tire themselves out! It also provides a way for you to spruce up your tank with a cute picture or color pop in the divider. This way you don't have to buy clings specially made for the back of an aquarium! I made this up myself!
Feel free to ask questions or show pictures of this idea in your tank if you use it!