Hello Gemma! Not sure if you can find something made of inert plastic at craft store that could be used as a frame for nedle point canvas. Then could use a couple rocks to prop it vertically.

Another option would be a visit to hardware store and get some window screen kit or buy the frame and plastic screen separately. Try and find the aquarium divider (Lee and Penn Plax make them); depending on tank size it might be your lower cost option. Good luck !