These keyguards fit the ArithmeType Math Keyboard.  They come in a set of three separate keyguards, one for each of the three keyboards, and they press on to the keyboards just like the keyboard covers, so you can use 1, 2, or all three at a time.

ArithmeType keyguards come in two colors and two thicknesses.  They are made from unbreakable polycarbonate and are have a lifetime guarantee against breakage.  Order 1/8" thick polycarbonate to have the surface of the keyguard slightly above the top of the keys, making for shallow holes and requiring a less deliberate touch.  Order 1/4" polycarbonate to raise the surface of the keyguard 1/8" above the top of the keys, making for deeper holes and requiring a more deliberate touch.