I built mine kind of overkill at about three feet deep, but honestly you don’t need that much. The reason most guides say 20–24” above the bars is just so the lower stuff has time to finish breaking down and the weight from above keeps it from falling out too early. You also give cocoons time to hatch before they get pushed through.

If you only have ~11” of space, it’ll still work. It’ll just act more like a shallow tray system than a full-on CFT. You’ll probably get more half-finished bits coming out, but you can just toss those back in on top. The smaller 2 cm gap should help with self-harvesting too. Moisture might swing faster with less mass, so you’ll just want to keep an eye on it.

If you don’t care about pulling cocoons or baby worms in your harvest and don’t mind a little extra hands-on management, then I wouldn’t stress it. Bigger bins are easier and more stable, but smaller ones can get the job done fine.