This is how Revelate started, sort of. They actually started by designing custom frame bags. Now they use all of the knowledge they acquired from their beginnings to made off the shelf bags that fit most hardtail bikes (they also make smaller bags that can be used within the main triangle of dual suspension bikes).
These bags are very well thought out. They are super durable, including zippers that feel virtually impervious to the elements. There are enough harness points to make sure the bag stays in place and doesn’t bind when you’re pulling the zipper closed with a stuffed bag.
There’s an outside, zippered pocket, an inside mesh pocket and divider, a port to allow for a hydration tube or battery cable to pass through, a flared front to allow for more volume near the head tube, and more.
Once you use one of these it’s hard to understand how you survived without it. Cost is $200 plus taxes. We have most sizes in stock at the time of this posting.