When I created my Sphere/Ring worlds, I had to create a shipyard ship and move it to the location that I wanted the ring/sphere world. Then I was able to build starbase sized vessels with the needed components. Once you have all the components built (it will take quite a few starbases to have everything you need) then you will be able to go to the ring/sphere world button in the stellar event (I think that is where it was) and create it.
It helps to have a bunch of ship yard ships working on multiple components, otherwise it takes FOREVER!
There is a good step by step description in the thread :
Ring/Sphere world construction
and another thread
Ring/Sphere 2
Hope all this helps
