If I remember correctly SMB charges about $500 for their bed. I elected to make my own. Figured I could make 1-2 mistakes before I reached the $500 dollar mark.
Interesting about the "span rated" plywood. Did not know of that before. Will ask the guys at my local lumber yard some time about it.
I made our bed platform out of a sheet of 4' X 8' 3/4 inch cabinet grade plywood. Cut it down to 41" inches wide by 6'6" long. Did the long cut FIRST and used the 7" wide scrap to make the platform 1.5 inches thick at both ends. 7" wide at the foot (where you climb up) and 4" wide at the head of the bed. Do not know if you have a power roof or a manual roof. The manual roof is very sensitive to the amount of weight you can hang from it. Last time I weighed our bed in came in at about 80#'s. That leaves about 40-50 pounds of lift still on the roof when you have the bed suspended out of the way.
The bed supports our weight and a grand kids fine with no noticeable bow or springiness. If hung for the ceiling for several days though (van home and working on it) I do see a slight bow developing in the longitudinal direction. It corrects itself after the bed in on the rails for a day or two.