The PiBox case apparently can only house two 2.5" SATA drives, not regular 3.5" hard disks commonly used for cheap mass storage. 2.5" hard disks seem to peak at 5TB per drive so the potential capacity is limited unless you go with SSD at much much higher cost.
The PiBox's 15W power supply leaves very tight margin for its components: Pi4 peak power consumption is about 6.5W, two SATA SSDs will peak over 5W each as well, so you should watch for power for stable operation.
I couldn't find a 2.5" 2-bay NAS model so the form factor isn't identical, but the closest match is probably QNAP TS-230 w/ similar 4-core ARM A53 SoC supporting two 3.5" drives for $199. The obvious downside is that RAM is capped at 2GB and that might be a deal breaker if you want to run lots of apps, but for entry-level storage needs it should be sufficient.