If it helps - there is a ap for this! The ap does not allow you to schedule a interview, what it does do it pop up with notifications when there is a available day/time at your pre - selected locations where you are willing to travel to interview.
We applied - my husband had his conditional approval (or whatever it is called) and ability to go interview in 24 hours. I'm still waiting, and it's been a good month. We wonder if my husband got approved so fast due to his already being cleared for base passed on Ft. Meade/NSA & Ft. Belvoir - but who knows. I've read it can take upwards of 9 months to hear back.
The app is called Global Interview. Again, it doesn't let you schedule - just alerts you to openings. And I was told by a border agent that they often release openings on Tuesdays, sometimes Mondays. No idea if it's true or not...just passing it along!
We have a international flight later this year, might do the walk-in interview when we arrive, although honestly the last thing I feel like doing is waiting around after a long vacation and a 10 hour flight.