Needing 12 and 5 Volt power the SFF has only SATA power cables and of course I don’t have any SATA sockets. But I do have a junked laptop drive and cut out the SATA socket. The PCB is so thin ordinary scissors works here. This hacked socket is epoxied to the perfboard, and the rest of the components are hand-wired. 

This perfboard is zip-tied into an old ear-bud case, ready to be stuffed inside the computer.

The IR sensor has a little piece of heat-shrink over the IR LED, this keeps down light reflections inside the computer case. The sensor perfboard attaches to a strong magnet with a piece of steel wire (a paperclip will do.)