I just bought the Rambo medium turnout for my ID cross - who has pretty broad shoulders - and I love it. light to put on, keeps her really snug even though she has just been clipped - and it doesnt move even if she has had a good roll. DEfinitely recommend (I had the lightweight one beofre for several years, and still looks like new).
My friend has the rambo and it is a really good rug.
However She wasn't impressed the fillet string broke in the first week and caused her horse to wiggle out of the rug! Just one of those things but it is an expensive rug! It also has a seam in the neck which has cause the mane to really thin. But those are the only negatives.
My retired horse lived out 24/7 in a Rambo plus last year and it didn't budge or leak. It has cleaned up easily and looks almost new. You will need quite a big size though as I think they are pretty snug around the neck/shoulders - mine is 3/4tb and she still had to have a size larger than she measures. You could try the style made especially for larger horses, as mentioned by Stinkerstonkersmum.