I was thinking of adding a 4-8 bay U.2–U.3 backplane behind and mounted to one of the mid-wall fans.
Would print something like these:
https://www.printables.com/model/248897-19mm-spaced-u2-25-ssd-bracket-for-4x-drive-backpla
https://www.printables.com/model/534247-120mm-fan-to-6-25-ssd-mount
https://www.printables.com/model/817781-4x-25-hddssd-case-housing
Idea #1: SFF-8654 (SlimSAS) 8i to 8x SFF-9639 (U.3) cable screwed into 3D-printed drive sled (like the HL15)
issues: finding short cables (<.75m), finding SFF-9639 male to female adapters WITH screws, power management
Idea #2: 8x SFF-9639 (U.3) backplane with 3D-printed enclosure on top.
issues: finding backplane with proper data and power in for clearance, space and fitment concerns, drive orientation in-line with fan flow
Will likely post back here if I ever manage to do it.
Keep us updated.