And so, to braces.
The top is not flat but is dished with a radius of c. 25 feet (I didn't know that before making one but this is pretty universal!) Curving the bottom of the main braces and pressing those down into a radius dish produces that shape.
First was to cut out the brace basic shapes:
Then to radius the bottoms so they fit snugly in their respective positions in the radius dish:
Then to glue them to the top, you have to hold the braces down, pressing the top down into the dish until the glue has fully set. To do that, generally builders use a 'go-bar deck' - two squares of MDF or chipboard, secured by threaded rods:
..and then dowels or fibreglass rods used to press the braces and top into the radius dish:
And when it's set, the braces hold the top in the 25' radius spheroidal shape: