This is my 'OneInTheChamber' plugin that I'm using on my server, released upon request. However, please note that I may not provide ongoing support for this plugin.
Commands:
Default start in 30 seconds (I use it on my Jailbreak server) !oneinthechamber (same as !oinc)
Default start when using the command (I use it on my AWP server) !setoneinthechamber (same as !setoinc) - Insta start.