Today, there is no shortage of SBCs out there, and intel has released some pretty powerful x86 based ones, like the minnowboard max 2.
On the market at this very moment, Western Digital is offering an external hard drive that has an interesting enclosure. (See Western Digital MyBook 3TB and 4TB models) This is basically just a little triangle shaped USB to SATA adapter attached to a standard 3.5 inch SATA HDD, which is itself mounted on 4 little rubberized pegs, held into the enclosure via some little receptacles for the rubberized pegs.
Now, the hardware hack.
I bought one of these late one night (way after midnight after all more reputable sources of computer parts had closed) just to get the HDD inside, as I needed a replacement RIGHT NOW. (Got the 4TB version. 3Tb drives have terrible failure rates. It was a 4TB WD Green series SATA drive. Not splendid, but it serviced.)
That left me with the shell. For awhile I left it to sit around and ignored it, but the more I looked at it, the more it just screamed to have something done with it.
The drive kit came with a 12vdc wall wart that can put out about 30W of juice. The enclosure has cutouts for the 12v barrel connector, the "USB3.0 HDD style" connector, and a lockstrap hole.
Minor modifications with a dremel tool made the USB slot into a standard USB sized opening, and the lockstrap hole large enough to accomodate a mini HDMI port.
Inside, I took a 2.5in to 3.5in bay adapter, put the rubberized pegs on, then marked mounting points for a minnowboard max 2 with a sharpie marker, drilled them out, then attached standoffs using a combination of small back-facing nuts and washers. In the 2.5in bay, I installed a 2.5 inch SATA HDD.
The minnowboard is unique among SBCs, because it has a real SATA interface on it. It is a dual core intel atom system with intel integrated video. Whoopy freaking do, except for the fact that it's total TDP is around 6 watts. That's low enough to run without a fan, and well within the 30W the DC supply that came with the drive can deliver. The problem is that it needs 5vdc, not 12vdc. Easily fixed with a DC-DC power converter.
Long story short, I found that there was enough room inside the enclosure for the HDD, the minnowboard, extender cables going to the port openings from the minnoboard, an interal USB2.0 hub for things like WiFi and Bluetooth, the DC-DC power converter, and all that jazz.
It makes a very snazzy looking HTPC box.