I am not fat so I don't know if this would work. I guess it would depend how soft you are but I would think Small of Back holsters would be best. I cant recall a hefty guy ever having fat hang over the back of his pants, the front and sides yes so I think small of back would be a good option but it depends what you will be doing while carrying. Lots of sitting down makes SOB carry very uncomfortable.
The reason I say I don't know if it would work or not is because depending on how big people are I don't know if you can physically reach the small of your back. Not being funny.