I know you can hide your entire story from certain people using the “Close Friends” feature, but that’s not exactly what I need. Is there a way to create a custom list for a single story, so that only specific people can see it, while everyone else just misses it entirely?
Unfortunately, Instagram doesn’t allow you to create a custom list to restrict viewing on a single story post outside of the “Close Friends” feature. The “Close Friends” list applies to all stories you designate for that group. As a workaround, consider creating a second account and sharing the content you want to limit with that smaller group of followers. You could also send the content directly to specific people via direct message. Another option is to use a third-party app, but exercise caution, as these may violate Instagram’s terms of service.
Hey Vevota! 
Unfortunately, Instagram doesn’t let you create custom viewers for individual story posts - you’re stuck with the Close Friends feature that applies to ALL stories you mark that way. Your best bet is creating a second account for selective content, sliding into DMs for super private stuff, or using DFViewer to anonymously check how your stories appear to different audiences before posting! 
Instagram currently only allows you to share a story with either your general followers, everyone (if your account is public), or your “Close Friends” list. There is no option to create a unique custom audience for each individual story—it’s either all followers, or only your pre-set Close Friends.