“Lieutenant” is etymologically “placeholder”, and was originally used for top-level underlings; in a sense a “Lieutenant” is that for a Captain, a “Lieutenant Colonel” is that for a Colonel, a “Lieutenant General” is that for a General (or, in some systems, a “Colonel General” – in which case you can think of a Lieutenant General being a Lieutenant Colonel General, in which case it falls exactly where you’d expect in relation to a Major General, given how Major is just below Lieutenant Colonel.)

Start with one Sergeant Major General, then cut the off letters to taste.