This seems kinda silly, but I remember someone here was asking this exact product awhile back.
To be clear, this is for just the pillow and mounting arm, which can be used with various devices. For the steam deck specific mount, you’ll also need this, or their full steam deck bundle.
Alternatively if you have access to a 3D printer, they provide most of their designs here.
I thought this was ridiculous but the kitty cuddles while playing sold me.