I cut off all the buttons and used the hot-melt glue (so they'd be flexible) to glue them in place. Needless to say, the old radio is toast.

Fine stealth job!

One thing though... I hope you used the high-temperature version of the hot melt glue. When I did the stealth mount of my radar detector, I originally used hot melt glue, not realizing that the car interior rises above the melt-point when parked outside on a hot summer day.

After all the glue melted away, I re-did the work with RTV silicone....