Another possible choice:
http://cgi.ebay.com/PORTABLE-HDD-STO...item439ccc9ac3
Designed for 3.5" hard drives. Height & width look good, and thickness exceeds the JBL's dimensions.
Page cites material as "PP". I hope that is "polypropylene " instead of something else (grin)
On second thought ... don't know if those dimensions are "outside" or "inside". A 3.5" HD is a bit narrower than is the JBL (4" rather than 4.25")