With an STX board mounted to the top of the dock (standoffs attached to the unit directly).
Hotswap unit looks to be 1.6 x 5.7 x 5.9 in, STX mobo would be 0.1 x 5.5 x 5.8 in, looks like they're almost made for each other. Looking for a way to change the M.2 slot to a a Raid controller/port multiplier with an SAS cable plug. OS could be FreeNas on a USB thumb drive.
I had had an idea for such a thing with up to 20TBs of total storage and using a Mini-ITX board and Flex-ATX PSU. The idea was sound but I scrapped it due to the cost of 2.5" hard drives costing 30% to 50% more per drive for the equivalent amount of storage. This was what I had in mind: