Here is a sketch for a butterfly cage. I realise you guys already using similar design with PVC but I could not find small enough PVC joints and aluminium (aluminum) could be somewhat stronger.
To build the cage, get eight 15 mm elbow corner fittings used for automatic watering systems. Aluminium or plastic curtain tubes of the same or slightly smaller diameter will fit tightly into these fittings. Straight fittings can be used to extend the vertical sides of the frame.