If you are putting together a Blu-Ray set as a gift, wouldn’t it be better to get some actual Blu-Ray cases from someplace like casetopia.com? The prices are pretty reasonable, and it would look better, in my humble opinion.
That is unless you are doing a burned set of Harmy’s DVD-5 releases. In which case, your DVD cases would indeed be more appropriate. If you poke around the site a bit more there are other designs already proportioned for DVDs, like this superb “Faces” set posted here.
It’ll probably be pretty difficult to find a Blu-Ray cover that was designed for a DVD sized case, though.