It's by far the easiest keyboard mod I've done. I've done it to three boards. Open the keyboard up. Then you buy some drawer liner from a home good store. I got mine at Target for $2 for a big roll. Cut it to the shape of the case, making holes for the stand offs as needed. Lay the drawer liner into the case. Then put the plate/PCB back into the case, sandwiching the liner between the case and PCB. Then replace the top cover.
Takes about 15 minutes and costs less than $2.