It doesn't work as great without dynamic lighting, since it won't light up the surrounding area, but it looks quite convincing.
You can push the fire charge around as needed with pistons and blocks but it's quite finicky. Inside the trapdoors, I place a yellow stained glass block, you can freely place blocks in the same spot as the fire charge, and it also won't ignite any blocks and you can freely walk through the same space in survival without taking damage.
It's however not as easy to remove it, once it's been created. The only way I know how to remove it, is using commands.
/execute as u/p run kill u/e[type=small_fireball,distance=..10]
This command will remove all floating fire charges within 10 block radius.
by _Cecille