Get a set of cork borers. See the pic. Essential for anyone who wants to make custom soft washers. Available from Amazon, eBay and any scientific supply house. Works for rubber, leather, and soft plastics (vinyl, polyethylene, polypropylene, Teflon, etc.). You can do the big cut first. Then if you need a rectangular slot, use a utility knife or scalpel to cut it. If you need keyhole slots or slots with rounded ends, use smaller cork borers plus a utility knife. If you need to make a lot of soft washers, you can buy a more expensive cork-boring machine that has a crank handle or motor.
Mike in California