Go to an office supply store and they have clear mats that are made to go under your chair. They have little "spikes" on the back to make them stay put on the carpent. I've had mine for about 15 yrs so they really hold up and are only about $30.
Sorry, just read you have this; why not try to get another one that is like one of the ones you already have so it's the same height. Or you could get a large piece of vinyl floor covering.