We do hundreds a year; usually over a jingle package of pre-programmed 30, 60 and 10 sec beds. The donut is critical. Make sure you have copy which fits and dosen't "push the in and out.
Also, if it's a real spot to be placed, it needs to be 29.7 or so. No media folks I know will or can use a 30 that's not under slightly.
Goo luck. Check with the media to see about the preferred deliverable.
To be effective, the background over the voice-over can't bee too busy, and obviously, volume must be right.
Radio packages are FUN to work on.
Russ