Other World Computing, otherwise known as OWC, has unveiled a new product dubbed Multi-Mount. There are multiple different versions of the Multi-Mount that are used for different mounting needs.
All of the mounts allow you to put storage into bays that weren't designed to hold an SSD or HDD. There are versions of the Multi-Mount that will let you store a single 3.5" HDD or a pair of 2.5" SSDs or HDDs inside an optical drive bay.
All of the mounts are made from aircraft grade aluminum and have an open back to keep the drives cool. The drives are also mounted with floating rubber suspension to reduce noise and vibration. The mounts range from $18 to about $50.