An extremely dense Rubber based sound dampening product that is specifically designed to absorb sound. This is an excellent material if you are looking to insulate your keyboard in the best possible way.
This comes in an 450mm x 200mm sheet, and can be cut to easily fit inside any keyboard case.
Please ensure that your keyboard has 3mm of true clearance at the bottom of the case between the case shell and the PCB, as this material cannot be compressed like foam. This material is also adhesive on one side, but can be applied either way inside the board.