Hi, I recently had a similar problem. See the screenshots below for my blueprints. (the initial inspiration came from this post (Can Exclusion Zones Be Updated at Runtime? - #3 by janine)
The cube is a static mesh actor I place in the world and with the local bounds I’m calculating the four corners of a square. I’m using local Unreal locations. Neither world location nor lat/long.
The actor I call Get Actor Transform on is the parent of the Cube mesh.
The order of elements in the Spline Points array caused me some confusion so maybe look into that as well.
Afterwards I’m getting the CesiumPolygonRasterOverlay component from the Cesium3DTileset that’s placed in the level and I add the previously spawned CartographicPolygon to the Polygons array.
Oh and if you plan on removing the polygon during runtime as well and you run into issues there, maybe check out my thread where I dealt with this problem 