SSD Stackers (as well as the old "Dual SSD Stackers") are meant to allow you to stack and mount two (or more, now thanks to SapperSquid's suggestion) 2.5" drives (SSD or HDD) together, and then be mounted in desktop computer cases that have mounting holes for only a single drive (where there is still clearance for two 2.5" drives).
PRO TIP: Always start by mounting your top drive from the bottom, and stack from there... I might try to get some instructions put together later to illustrate this.
ORDER YOUR SSD STACKERS AND MULTI-STACKERS HERE: https://www.ryang3d.com/shop/ssd-stackers/
2-1-16 - Finally got around to testing the new "infinistackable" design (9mm widehole version) myself, and I'm pleased to say it works great! I only had spare 2.5" HDDs laying around to test, but this will absolutely work with SSDs (recommend the 7mm version for SSDs). Have a gander at the newly uploaded image of the new design shown in red... I'll try to post more images/info later.
11/9/15 - New files (ssd_stacker...stl) are currently untested, but should be infinitely stackable! Added new stacker design with SapperSquid's suggestion, and a couple other tweaks implimented. New filenames now reflect drive sizes, and also made a second version that has slightly wider holes for screws to fit through easier (hopefully). I will try to follow up in the next few days with a photo of more than two drives stacked using these.
Might try compacting things even more in a future iteration... for those that might need to fit more in a smaller amount of space. Current design allows for better airflow however.
The original "dual_ssd_stacker_final_flat1" files are meant for 9mm drives, and the "dual_ssd_stacker_final_flat1_7mm" files are meant for 7mm drives.
12/11/14 - Just tested the 7mm stacker with two 1TB Samsung EVO SSDs, and they mounted beautifully. Furthermore, the two "stacked" drives mounted just as well into the HDD/SSD sled that came with the computer case I am currently building into.
Print settings are not super important, just make sure the parts come out nice and strong!
Will come back later to write this up more, but for now just make sure to start by mounting your first four Stackers to the bottom of your top drive, and stack on from there!