Rust Radiation zones that shrink with time

For those who don't have experience or know about Battle Royal im going to break down exactly what i'm looking for...
create a zone with say 100 radius it stays at 100 for 3 minutes after the 3 minute timer a popup is made onscreen saying "The playable area is decreasing in size." and the zone then starts getting smaller in size anyone that is outside of that zone as it decreases in size is given Radiation.

 

Sounds quite easy. Using the api of ZoneManager with some basic comand-interface and timer.