This design takes into account the weight distribution of a hot tub. The weight is mainly in the center, in a 4- 5 ft square, and on the side walls. So using lots of double joists looks like a waste of wood. How about building a normal square frame, but have only two diagonal joists. Viewed from above, it would look like an "X" in a box. This would sit on five 6-inch concrete piers, four corners and one in the middle, where the diagonals meet. I would then use either 5/4 decking for the floor, or maybe some PT plywood. The original proposed using single 2X8's for the joists, but I'm thinking of doubling those.
My other option would be a gravel bed. But my problem is that there's a slope of about 12" over the 8-ft span. I don't want the gravel bed sliding down the hill. Might be an interesting slide if I'm in the tub at the time, but not worth the effort. Digging a level 8X8 X 1+deep hole, even if it's not that deep at the lower end, might be more work. And then I still have a 12-inch high gravel mound at the end. So now I need a retaining wall. The deck looks more attractive by now.
So thanks for any thoughts on this.