The city - a modern, sterile 'utopia' (with the undercurrents of government control and suppression, of course), that can be current tech, or future tech so long as the day-to-day lives are in current tech. Lots of surveillance, propoganda, etc., but people seem to go about happy for the most part. A typical trope. Most buildings are ground-plus-two (G+2) stories maximum, except a few civic and institutional buildings are up to G+5. Isolated population of about a million "happy" denizens.
The Trophy - The city in my story is commemorating an event. My design is a sturdy rectangle measuring 150m x 150m and is about 10m thick (I can be flexible on this). I don't want to have any protrusions off of it, and I want it to appear to be solid black stone. The problem is that it is also is about 350m above the city.
It can float around a bit, but I don't want it to have a big air bag or anything like that and I want it to be silent to the people on the ground.
Now I'm at the point in the story where it needs to be explained with a minimum of hand-waving. Can you explain how my urban trophy hover around for years above a city?
EDIT: It should always appear to be a floating stone for decades or centuries; no-one must know it is a mechanism (e.g. - nobody can go near it once it's aloft).