I basically have one (based on DeskMeet X300), although it runs on Bazzite.
Dunno if SteamOS would've worked back when I set it up.

I preferred Bazzite, because it provides full disk encryption out of the box whereas there's only a kind of workaround for encrypting the user profile on SteamOS - unless you want to risk breaking all by SteamOS updates.

With current prices building 3rd party Steam Machines is about as appealing as it is for Steam releasing Steam Machines.
I would't have that "Steam Machine" if it weren't for repurposing the DeskMeet X300 that I already had and which I no longer needed for its formerly intended purpose (it was in the end a waste running the Proxmox Virtual Environment there and I moved the PVE to a Dell Wyse 5070).
Well, that's gone quite off-topic. What I wanted to say was simple:

current hardware prices are such a pain in the ass and make building Steam Machine no fun!