Suggestions for gig-economy scheduling software?

Hey everyone! I'm working with a startup now that offers gig-economy work. We have lots of "agents" on our platform that can work whenever they want, for as long as they want.

Given this flexibility, they don't really need to schedule anything, but we'd like to have our agents choose "shifts" when they'll be working, for accountability and to give the job some more structure.

I've been looking for some software that would let us do this, but a lot of employee scheduling software is designed so the managers can push out new shifts, and employees can pick them up. This won't work for us because we aren't concerned with overlap or having multiple agents in the same shift.

Does anyone have any suggestions on software we could use for this? Here's basically what we need it to do:

1. We can add time blocks, such as 9am - 11am, 10am - 12pm, etc.
2. Agents can view these blocks and choose the ones where they'd like to work
3. Other Agents can select the same time blocks, no matter if someone else has chosen it or if there's overlap with another shift.

I'm kind of imagining it like scheduling staff at a restaurant, except we can have an unlimited number of waiters during any given shift, and we don't care if there are no waiters available for certain parts of the day.

Thank you!