Here is another item that may seem like a stretch for a website that carries "tools that teach".
Paraffin wax blocks actually make a great barricade for studying the wave patterns in a ripple tank. Heavy enough not to be moved by a water ripple and large enough to be an effective barricade. Two blocks will also sit nicely side by side to extend the barrier length. Paraffin has an indefinite shelf life and will not dissolve in or absorb water. Sold in boxes of 4 blocks. Each measure 5"x3"x2"