you have to replace the whole kit-n--kaboodle. i believe it is now around $700 >
mine cracked a long while ago, but i didn't do anything about them and now the lcd screens are messed up.
somethign that might work for you if the outer rings are still in tact is this.
get some plexiglass and some clear silicone caulk.
just cut out the clear plastic cover around the inside of the black part. cut a piece of your plexi to the right size and run the caulk around the edge. make sure you don't use too much or it might drip down on your gauges and do something bad to them (plus it will just be unsightly)
let it dry and check to make sure that the top part hasn't come loose. if it has then caulk it up.