Are there any manufacturers today, that build PC cases specifically for Sleeper PCs?

To what degree do you mean?

You can certainly still get modern giant monolithic tower cases with CD and floppy drives in them.

Not so much a new Atari computer case, unless you want to 3D print one yourself, e.g.:
https://www.yeggi.com/q/atari+case/
https://www.thingiverse.com/thing:2796615

@Zaku
I am talking about something like this and this.

… except not like those because they’d be new construction, for making fake sleeper PCs?

In any case, I haven’t except for hobyists either refurbishing and upgrading old computers or making new cases for themselves, or doing things like in the link you posted.

I would think though that you could get a clean new metal tower case designed for modern boards, and then plug in the face plates and/or maybe attach the framing of an old machine.