The Asrock N100DC-ITX uses a 19v power brick so no separate PSU to power the drives. It comes with a 4 pin cable that splits into 2 SATA power.
The case I chose, Silverstone SG13, comes with a 3.5" HDD bracket but you need to mount the drive belly up so you can screw it in. The second drive I want to put where the PCIe slot is, found a nice 3d model for that. The thing is, the power cable would be too short and the drives would be too far apart to reach both.
So I removed the HDD bracket and cut 2 L-shaped aluminum stripes and I screwed the drive belly down. This way the cable now reaches both drives. I know, I could have cut the cable and made it longer but I prefer to avoid playing with power cables if I can.
Turned out quite good. Wanted to share it with other people. Now I need to find someone to 3d print the PCIe HDD holder and find some rubber stuff to put under the L-shapes to dampen the sound of the drive. This is the 3d model, https://www.printables.com/model/385403-pci-slot-hdd-bracket but V2 with the nice feet.
PS the drives in the images are broken so I just used them as dummy drives.
That's about it.
Unfortunately it doesn't say in the manual. It only says that if you use 4 HDDs you should use a 90W DC 19V power brick. I already ordered one, my confusion was if the 4 pin cable supports 2 HDDs but I guess since the board has 2 SATA slots and cable also has 2 SATA power connectors. I plan to most likely use 2x WD Red Plus 8TB, they use 6.2W (load), 4.1W (idle)