I really like the Mountainsmith
Day lumbar pack with strappets.
It's a large fannypack with additional should straps for stability. Since it rides low, it helps a lot with back ventilation.
If you use the compression straps better than this guy, it snugs down very compactly, but still expands a ton. Ugly teal colored ones from the 90's pop up on eBay for 25 bucks or so.