Anyone doing a Kentucky Derby office pool?

Thinking of throwing something low stakes just to get people interested/invested in the race around the office.

What's a good way to do it where everyone has an equal buy-in and participation isn't capped at the number of horses?

Anyone do something in their office/group of friends that seems to work well?

Admins feel free to move this to More Sports Board if it fits better there, wasn't sure.