Im having the same issue. We tried the inflatable ring, the neck brace thing, and three different cones, but our dog can still reach her foot. This is the first cone I have found that comes in lengths longer than 1 foot (only for the XXL size), and Im going to order it. Im afraid that the neck hole will be too big though, because my dog is only 60lbs and not at all xx large.
https://www.amazon.com/Original-Comfy-Recovery-Collar-Removable/dp/B007K5BR3C?th=1