You would use one of these SSD Mounting brackets. (inside the blue 3.5inch tray) But if possible, it would be better to leave that bay empty for better cooling. USB-3.1 (external, with USAP ) is effectively as fast as SATA. Or just go ahead and get a Synology NAS. With a 4 bay, you have plenty of room for expansion. When loaded, makes for a nice RAID-5 solution (if you so choose).  
I would like to install a 2.5" HDD in the 3.5" caddy/slot.  I already have 2.5" drives occupying the two remaining slots. The reason for going with 2.5" is for lower noise, power consumption and heat.  I also had a poor experience with a faulty 3.5" drive purchase. What do I require in terms of a caddy?  At the moment there is a blue plastic caddy for the 3.5" drive.  Does that get entirely replaced or is there an adapter that fits within the existing plastic caddy?
Aurora R11, HDD Bay Conversion