UptimeRobot
UptimeRobot checks websites from outside and alerts when they fail. It has a free plan with 5-minute checks, which fits small sites on fortrabbit.
About UptimeRobot
UptimeRobot checks websites, keywords, ports and cron heartbeats, and includes status pages.
Pricing
A free plan for personal use with 5-minute checks, paid plans with shorter intervals, see UptimeRobot pricing.
Integration
Add the domains in UptimeRobot. fortrabbit needs no setup.