Long story short, I'm building a home lab cluster and added two Intel NUC 11s to my existing NUC8. Part of this was to get a Ceph cluster going across them and while the two new NUCs have SSD bays, the NUC8 was a Slim model that did not have the extra bay. It does, however, have the SATA and power connectors... enter 3D printing.
I found a model to extend the NUC with a drive mount on Thingiverse but after printing it, it turned out that there were a bunch of problems: first, it called for M3 bolts but the NUC uses M2.5 for the bottom plate. There were also a bunch of alignment issues and the images on the listing were not the same as the models. This meant I'd have to take on modeling one myself with the goal of getting something as slim as possible. I found a couple listings for a cable harness that has a right-angled SATA, the correct power connector and a short cable to the correct mini-SATA SSD connector, so it all came down to how slim I could make the case to fit everything and be easily 3D printable without supports. Thankfully, Intel provides highly detailed STEP files for each NUC so I had perfect dimensions to start from.
Here's what I came up with:
Overall it came out way better than I expected, given the time I put into it. The shell piece mounts to the NUC, the drive frame holds the drive, manages cables and mounts to the shell, and the original bottom case mounts to the drive plate. The whole thing, buttoned up, is only about 10mm thicker than a NUC "tall", so it doesn't even look out of place from a distance. The lower right photo shows it with my NUC Rack Mount adapter attached and ready to go back on the rack.
The most difficult part of this turned out to be the mounting system, since the NUC slim has some very tall bolts in it:
There's just enough room to use new M2.5 bolts to mount the shell and not interfere with the case bottom mounting to the drive plate. All hardware is attached with heat-set inserts, M3 for Non-NUC connections.