Yeah, a week late to the party but if you are not tied to using a 3.5", often called Large Form Factor (or LFF) Hard Disk Drives and a 2.5" (SFF) HDD could still meet your needs, consider using a HP carry handle to house the HDD as outlined in this (and its referenced) post.
Such a carry handle can be usefull for other reasons. Though the carry handles are more expensive than a couple of simple "U" brackets, they are much more robust and less likely to cause vibrations and noise as compared to those simple "U" bracket solutions.