You can't beat polystyrene except its white and cockroaches eat it.
Use some PVC tape (like electrical tape) at the front edge of polystyrene board to secure to stand. This will stop the polystyrene sliding back when you push the tank on. Then pull the tape out when your done.
Another method is to use PVA craft glue to fasten the polystyrene board the day before as it takes a few hours to dry.
If you want to make the polystyrene less visible, you can paint it black, use a black marker pen or tape which is also good to hide the waterline at top.