Oh, there are plenty of big guys here. Your build must be just different enough to make it an issue. I'm 6-5 and notice the cavity down the middle but it doesn't feel uncomfortable at all. My shoulders are actually above and outside the bolsters.
Try what @
humfrz recommends. Maybe start with a good stiff sofa pillow or cut out a strip of memory foam. If it seems to work, you can take the plunge and strap a good custom device in there.