Just curious, why not rely on a knot and instead rely on hardware? I don't have ice diving training, but in climbing (even in ice climbing), we prefer tying directly into our climbing line(s) from the harness as opposed to having a carabiner in between, primarily to reduce the probability that the carabiner will be cross-loaded or inadvertently opened (which can happen even with triple lockers). We typically use a figure-8 follow-through (or some people use a bowline on a bight, though it isn't as widely accepted).