Yep, that's a bitch. It happen to me last year, I had it coming though. It was a bad part of town at about midnight, I didn't even think to take the faceplate off :doh: so stoopid...
They messed up the dash as well, cut it all up trying to rip it out, mine was in good, hopefully they damaged the radio doing it.
My advice to you is get a security system, don't get tinted windows, that'll atract them, buy a new a stereo and move on... all within buget, that'll be the tricky part. As far as I know there is no non-cheap mounting assembly.
My moms boyfriend once had a security system of his own, he devised these sharp metal shards and put them on the back of the radio, you were able to reach around on his car. One day he got into his car and found some blood under the stereo, the radio was still there but then he had blood to deal with.
One more thing, my friend mounted his stereo back into the assembly a few inches, enough to not be able to see from the side of the car, somewhat of a pain to push the buttons but it might save your stereo. Maybe make it so it can slide and lock in place, have it flush with the dash when your cruisin' then when parked unlock it, push it back, then lock it again, I dunno just some more creative ideas.
Sorry to hear that this happened at your work, thieves just bust your gut soemtimes.