The Kierr Flex 100 is made of aluminum and is lightweight. The adjustable plastic clamping surface also fits rounded and beveled window frames. You can easily tighten the window vent holder using the simple twist knobs. The rubber protection on the clamps ensures a secure fit on plastic, wood, and aluminum! Use in children’s rooms is at your own risk, partly due to parts that can come loose. Installing the Kierr on outward-opening windows is also at your own risk. The Kierr is clamped in place, so it can always fall down. Therefore, we do not recommend using it in areas where it could be dangerous if the Kierr falls. If you want a larger opening—for example, for pets—choose the Kierr Flex 200 or 300.