Mini-ITX 10-inch rack 1U shelf

You should print a small part of the model for test fitting

…because I cannot guarantee that my Mini-ITX board is the same as mine, especially in how far out the IO ports protrude from the motherboard itself.

Also, make sure you have enough vertical clearance! I had to really bend the cable of my ATX 24 pin to get everything to fit in 1U.

Assembly

Hardware required: 4 × M3*5 to secure motherboard to shelf unit, 4 × M3*8 or longer to secure IO shield to shelf unit.

1. Secure motherboard to shelf unit with 4 × M3*5 bolts; the bolts should cut their own threads into the plastic standoffs of the shelf unit

The IO shield is optional but good to have. Since most Mini-ITX's ship with a > 1U IO shield, you'll likely have to design it yourself.

1. Customize io-shield-template.3mf or io-shield-template.FCStd to fit your motherboard's IO
2. Before fastening the customized IO shield to the shelf unit, thread the M3*8 or longer bolts into their holes to begin cutting threads into the plastic
3. Finish fastening the customized IO shield to the shelf unit

If by some stroke of luck you have the exact same motherboard as I do, you can of course just use the io-shield-n100-nas.3mf. Here's the Taobao (basically, Aliexpress) product page for my motherboard:

Modified from Shelf_Hex.stl.