When someone decides not to fly at a time they've scheduled, the glider often goes unused because no one knows the spot opened up. Is there a scheduling system that lets you set email/text alerts on multiple times you're interested in?

We use aircraftclubs.com, which lets you schedule a "backup" reservation if someone else has a certain time, but that's doesn't work for what I'm looking for...it counts against the max hours I can schedule. The alerts would let me schedule open times and know about multiple booked times opening up..