Noob question: how can I make particle clouds sit behind the floor's horizon

New to c4d:

I've made particle clouds (Pyrocluster) and now want to have them 'in the sky' – so sitting behind a horizon line in my drawing. At the moment, I've added a floor object, but have tried a large disk, too.

(NB. I find the c4d sky object too complex at the moment, and I like the clouds I made so would like to try and make them work).

Just wondered if there's a simple way to do this?

Thank-you

Files:

https://we.tl/t-LmeSnnmiGQ