What you really want is made for HP by Foxconn in China, where many of the HP workstation parts are made. There is a fancy "tooless" one and a "needs screwdriver" one. These mainly metal adapters go in the HP 3.5" plastic drawers and convert perfectly from 3.5" form factor to 2.5" form factor. Thus, you can put 2.5" drives into your 3.5" drawers and they'll mesh perfectly at the rear of the drive drawer bay via the Foxconn "Blindmate" adapter already built in. These are better than anything else from my experience.
Go to eBay and search for 668261-001 and 651314-001.... the eBay sellers have mishmashed the pics together a bit but you'll get the idea. Here's two pics, of the two types:
That bottom pic is the "tooless" one that I actually don't like as much. It is shown there in one of the 3.5"drawers that your client already owns. With the tooless version the mounting is a bit more complicated. With the other version I just keep one empty in the top bay drawer, and it is quick and easy to drop in a 2.5" drive to partition/format/dink with via sliding it in (and not even screwing it down). For long term use I'd always screw it in or use the tooless version. Both have the same basic design with same strong HP metal and HP engineering. We don't use anything else now.