I remember seeing some of the very old silos here in Michigan farm country that did not have the perfectly half round tops associated with silos you see now.
They consisted of flat pannels, that numbered maybe 8 or more pannels, that sloped inwards and had maybe 2 tiers of them, the top tier sloping much more then lower tier and meeting in the center. Perhaps there was a very small cap over the center that I suppose you could say it was a 3'rd tier. It was a way to help seal the whole thing in the center, to the weather. The ones I remembered seeing were finnished off with old wooden shakes for shigles. The newer generations of silos had the round metal top covers that every one associates with silos. I cannot think of where they are now, around here but I am sure there are maybe a couple left,,, I just looked on GOOGLE and used their "Image" option and typed in "Old silos" ,,, there was quite a few pictures of em,, some had a rounded top that was kind of flat like a "Chinamans hat". that would not be real hard to make,, and looks kind of cool. Certinaly a lot easier then the ones that are perfectly round. The one I mentioned, would also look great since its also a focal point of a back yard because it "Looks old", and would be an interresting project to do.
But its your back yard and what you like,,, you like.. but maybe take a look on GOOGLE and maybe you will see something you like or atleast use as a image to work off from. Good luck.
It seems like every where I go,,,,,,,,,,,,,,,,,,,,,,,,, there I am...