just get a mesh object of the hemisphere with the flipped normals and drop it in the scene at 0,0,0 with a big scale…you can disable stuff like collisions etc as it will be just cosmetic…then assign some material with the look you want to achieve
if you dont know or dont have access to a 3d software (like blender?) I can provide you the hemisphere…its very simple to do!
Best,
Dany