OP here. Thank you all for the feedback. Will be on the babysitter hunt!

Reach out to your child’s former daycare or preschool teachers for overnights.

I think those are 2 different roles OP. I’d try and find someone for the childcare hours you need first. You can always ask if they’d be willing to do overnights on occasion. Do any of your friends have sitters they could ask for the overnights?
Close friends tbat might take your kid for an overnight in return for you having their kid for a date night?

Sorry, OP, that will be very difficult to find. My friends needed just the afternoon care for their kid, and they hired a series of college kids that would just quit without notice, whenever.

Professional nannies usually want full time work. From the point of view of the nanny, it's difficult to patch together several part-time jobs. You are also asking for a lot of flexibility that most workers don't have.

You will have trouble getting an after school nanny wanting only these prime hours but not more and will pay a premium for it, much less the occasional overnight ability. Maybe get an au pair who would illegally be ok with the overnights for extra cash.

You'll probably need to have 2-3 sitters on hand as one sitter to be available for different times may be difficult.

Single parent. For about the next 10-11 months, my job will require regular travel a couple of times a month. I am trying to figure out the best way to have childcare for a young elementary school aged child squared away for overnight trips. When I am in town (I.e. the vast majority of the time), I’d only need 10-15 hours a week of regular childcare, basically, aftercare pickup and a few hours on weekends. But I’m trying to figure out the best way to have someone reliable that I could depend on for overnight care if I have to travel out of town for a couple of days on short notice. Anyone employ a nanny/babysitter on such terms? If so, how did you locate them? Other suggestions/recommendations?