The flat deck area on the SRMN 600 is about 44" wide. Its wider from tube to tube but you have 44" of flat deck and then the deck angles up a bit.
I had the classic aluminum leaning bolster off the Hurricanes, its about 36" wide and fits OK on the SRMN. I wouldn't go wider than that otherwise it will be a tight squeeze to get around it. If I didnt have a single jockey seat I would be looking for a leaning post/bolster about 30" wide.
I just saw your other thread about consoles.. The large aluminum Hurricane console works good for the SRMN, although it takes a lot of the deck space and is super hard to find. There are custom aluminum consoles out there as well as readily available fiberglass consoles that will also be a good fit.
I might go with this one in the future:
CM Marine Products