Surly mountain frame bags are designed specifically to fit in the main triangle of your Surly frame. Their mountain bags are now using a ladder strap system, which allows greater flexibility in how you attach the bag to your frame. The ladder straps let you customize the fit and placement of the mounting straps so if you have some weird contraption clamped to your frame tubes like a chain keeper, cable pulley or some trinket grandma gave you, you know it won't interfere with the mounting straps on the frame bag.
Handmade by Revelate Designs in the U.S., these things are made of Dimension Polyant VX 42, a synthetic sailcloth. This stuff is durable, rip resistant, and tightly woven and laminated to make it highly water resistant (but not fully waterproof… Revelate advises using drybags inside if you risk riding in extended downpours or underwater).
Compartment access is controlled by quality YKK brand water resistant zippers. The large main compartment has a flexible divider inside, and there's a hidden pocket too. There's also a compartment suitable for smaller items you''ll access relatively often, like your map, phone, hard boiled eggs, uncontained pudding, marbles for playing marbles, your dugout, that ring you found that makes you disappear when you put it on, or whathaveyou.
Frame bags such as this are becoming more and more popular. They're light, reasonably voluminous, stable because they center weight low between your wheels and body, and they go places where racks and panniers are often troublesome, like singletrack.
Surly offers them in ten sizes for many of their off-road frames. Buy the bag that matches the size of your frame based on the handy sizing chart.
Note: due to the rough texture of the mounting straps, prolonged use may lead to scratched or worn paint. Those worried about keeping their frame looking pristine should immediately look for some other type of bag, or figure a way to protect their paint, such as duct tape. Or tatting a top tube doily. For example.