For me, frame bags are one of the essential components of a bikepacking rig. Getting stuff of your back is one reason, using the space made available by your frame is another, but the biggest one is keeping stuff handy on the bike. A properly designed frame bag lets you access water, snacks, camera, and layers while still pedaling.
The classic in the genre is the Jandd Frame bag. Very solid and affordable, but not a full frame bag, and the way in which the bag is not anchored solidly on the frame fore and aft makes the zipper a bit harder to get at with one hand.
Thus, the only real option is custom. Forunately, this is a fairly easy MYOG project. Here's the evolution of one I made recently for my Surly Karate Monkey.
I selected a panel loading design, with interior mesh pockets for tools and snacks, and stretch divider in the lower part to keep things tight (so your legs don't hit the bag. A full bag a little less than 3" wide is what I've found to work well for me. I use a pretty low q-factor rig on my bikes, so others might be able to get away with fatter bags.
Padding where the bag contacts the downtube and seattube is essential. It mitigates wear between the frame and the contents (make sure to remove your botle bolts, and put some electrical tape over the bolt holes), and keeps things from clunking. I use yoga mat sewn directly to the fabric. Cheap, easy, durable, and doesn't absorb water.
Next arrange your end pieces and sew on the velcro in the right spots. I like to cut the whole side from one piece of fabric, and I put the velcro in places that will keep the bag taut for ease of zipper operation.
Sew it together, flip it back outside out, and you're done! The orange flap is an overlapping bit of fabric for a drain hole. Sewing up the last side of the stretch divider was not easy as a last step. The seam is not straight, but should hold up just fine.
Installed. I'm not going to get a calculater and add up the cubic inches, but it's quite a bit. For a summer 24 I could fit everything in here, almost.
I prefer burly fabrics in most things, so I make these out of 100% Ballistics (16 oz a yard!). It's very durable and very waterproof, which is nice when water and mud from your tires are firehosing your bags. Weight weenies, especially those that don't crash their bikes regularly, could get away with Dynemma ends and silnylon sides.